audio-coreaudio

name	audio-coreaudio
description	Manipulate macOS audio at the Core Audio HAL layer: afinfo (audio file metadata), afplay (play), afconvert (convert format/rate/bit-depth/channel layout), say (TTS), SwitchAudioSource (change default input/output/system device — brew install switchaudio-osx), audiodevice CLIs (npm/Go variants), AppleScript via osascript to drive System Preferences Sound. HAL plugins at /Library/Audio/Plug-Ins/HAL/ load virtual drivers (BlackHole 2ch/16ch/64ch, Loopback, Background Music). Aggregate Device (drift-corrected multi-in) and Multi-Output Device via Audio MIDI Setup.app or AudioHardwareCreateAggregateDevice. Use when the user asks to change default macOS audio device, route audio between apps on Mac, install BlackHole, build an Aggregate Device, convert audio files with afconvert, or script Mac audio from the terminal.
argument-hint	[action]

Audio CoreAudio

Context: $ARGUMENTS

Quick start

• List input/output devices: → Step 1 (macaudio.py list-devices)
• Change default output: → Step 2 (macaudio.py set-default)
• Inspect an audio file: → Step 3 (macaudio.py info)
• Play a file: → Step 3 (macaudio.py play)
• Convert format/rate/bits: → Step 4 (macaudio.py convert)
• Text-to-speech: → Step 5 (macaudio.py tts)
• Build an Aggregate Device: → Step 6 (macaudio.py aggregate-create)
• List HAL virtual drivers: → Step 7 (macaudio.py hal-plugins)
• Install BlackHole virtual cable: → Step 8 (macaudio.py blackhole-install)

When to use

• User is on macOS and needs terminal-driven audio control.
• Wants to change the default input/output/system device without clicking through System Settings → Sound.
• Wants to convert audio files with Apple's built-in afconvert (more format coverage than SoX for Apple formats; handles CAF, M4A, ALAC natively).
• Wants to build an Aggregate Device (multi-input) or Multi-Output Device (mirror output to several devices).
• Wants to install / evaluate virtual audio drivers (BlackHole, Loopback, Background Music).

Not for: Linux (use audio-pipewire), Windows (use audio-wasapi).

The script exits 2 with a helpful message if run on non-macOS.

---

Step 1 — List devices

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices --format json

SwitchAudioSource -a underneath. Each line is <name> (<type>) where type is input / output. If SwitchAudioSource isn't installed the script tells you to run brew install switchaudio-osx.

---

Step 2 — Set default input/output/system device

macOS distinguishes three defaults: output (music/TV audio), input (mic), system (alert sounds, which can be a different device).

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "MacBook Pro Speakers"
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "Scarlett Solo" --type input
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "MacBook Pro Speakers" --type system

Device name must match exactly (spaces matter, quoting required). Use list-devices first.

---

Step 3 — Inspect and play files (afinfo / afplay)

afinfo + afplay ship with macOS. No install needed.

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py info track.caf
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py play track.caf
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py play track.caf --volume 128   # 0-255

afinfo reads every Apple format (CAF, M4A/AAC/ALAC, AIFF, WAV, MP3) plus anything the system's registered AudioToolbox components understand.

---

Step 4 — Convert with afconvert

afconvert is Apple's native format converter. Preferred over SoX for Apple-specific flows (CAF, ALAC, AAC for M4A/M4R, lossless 32-bit float).

# WAV -> Apple Lossless in an M4A
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input in.wav --output out.m4a --format m4af

# 24-bit 96k WAV -> 16-bit 48k WAV
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input hi.wav --output lo.wav --format WAVE \
  --rate 48000 --bit-depth 16

# Stereo -> 5.1 upmix (container must support it — CAF does)
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input stereo.wav --output surround.caf --format caff \
  --channel-layout 5.1

FourCC codes the --format flag accepts: caff (CAF), m4af (MPEG-4 audio), WAVE, AIFF, AIFC. Full list: afconvert -hf.

---

Step 5 — say + afplay TTS

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "Build complete"
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "hola" --voice Monica
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "saved" --output voicemail.aiff

say -v '?' lists installed voices. Voices beyond the defaults (Alex, Samantha, Daniel) require a one-time download via System Settings → Accessibility → Spoken Content → System Voice → Customize.

---

Step 6 — Aggregate Device / Multi-Output Device

macOS lets you combine physical devices two ways:

• Aggregate Device — sum inputs and outputs of several devices into one logical interface. DAWs see it as a single multi-channel device. Needs drift correction when the member devices don't share a word clock.
• Multi-Output Device — mirror playback to several outputs (e.g. speakers + BlackHole + AirPods). Output-only.

This wrapper opens Audio MIDI Setup and prints the clickable steps, because programmatic creation needs AudioHardwareCreateAggregateDevice from CoreAudio/AudioHardware.h (Swift / Objective-C). Use when wiring for automation:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py aggregate-create \
  --name "Studio Aggregate" \
  --devices "AppleHDAEngineOutput:1B,0,0,1:0" "BlackHole2ch_UID" \
  --drift-correct "BlackHole2ch_UID"

Member device UIDs come from list-devices (pass --format json for the full UID strings).

For a truly automatable path, write the Swift snippet that calls AudioHardwareCreateAggregateDevice — see references/virtual-drivers.md.

---

Step 7 — List HAL plug-ins

HAL plug-ins are .driver bundles that register virtual audio devices:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py hal-plugins

Typical output includes BlackHole2ch.driver, BackgroundMusic.driver, Loopback.driver, Aggregate/Multi-Output synthesizer drivers from Apple.

Global path: /Library/Audio/Plug-Ins/HAL/*.driver. User path: ~/Library/Audio/Plug-Ins/HAL/*.driver (rare; per-user installs).

After installing or removing a HAL plug-in, coreaudiod has to rescan:

sudo pkill coreaudiod     # wrapper intentionally does NOT run this

---

Step 8 — BlackHole (virtual cable)

BlackHole is the go-to free open-source virtual driver — works on Apple Silicon (DriverKit / AudioDriverKit) and Intel, 2ch / 16ch / 64ch variants.

The wrapper prints install instructions rather than installing automatically:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 2ch
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 16ch

Runs the install only if the user copies the printed brew install --cask line. See references/virtual-drivers.md for the full BlackHole vs Loopback vs Background Music comparison.

---

Gotchas

• Soundflower is abandoned. It does not load on macOS 11+ / Apple Silicon. Never recommend it. Use BlackHole instead.
• SwitchAudioSource device names must match exactly. Trailing spaces, capitalization, and the full name (including brand prefix) all matter. Quote-wrap everything. list-devices --format json gives you a copy-pasteable UID.
• There are two "audiodevice" CLIs in the wild. An npm package (node-audiodevice) and a Go binary. Both predate SwitchAudioSource and have different flags. Always specify the package/binary when pointing at them; default to SwitchAudioSource in docs.
• BlackHole, Loopback, and Background Music all use DriverKit/AudioDriverKit on Apple Silicon. Legacy kext audio drivers (Soundflower, old Loopback versions) won't load. If a driver supposedly installs but list-devices doesn't show it, SIP or kernel-extension approval is likely blocking it (System Settings → Privacy & Security → Allow).
• Aggregate Device needs drift correction when members don't share a clock. Without drift correction, multi-USB-device aggregates will desync by seconds over a long session.
• Multi-Output Device cannot be a system alert device. macOS rejects it — system sounds require a real output endpoint.
• afconvert fourCC codes are case-sensitive. WAVE with -f, not wave. Likewise AIFF/AIFC/caff/m4af.
• After installing a HAL plug-in, new processes pick it up; existing ones don't. Apps that cached AudioDeviceID values will miss it until restart. System Settings → Sound usually re-enumerates immediately.
• say runs through the system output by default, not whatever set-default --type input was last set to. If you need say → file, pipe through --output (AIFF) or afconvert afterward for M4A/MP3.
• AppleScript via osascript can hit Sound preferences via UI automation, but it's fragile across macOS versions and requires accessibility permissions. Prefer SwitchAudioSource for scripted default changes.
• CoreAudio HAL IDs are not stable across reboots. Use the device UID (string) instead of the numeric AudioDeviceID when persisting references in scripts / plist / DAW configs.
• BlackHole per-channel variants are not upgradeable in place. Uninstall blackhole-2ch before installing blackhole-16ch, or both drivers show up simultaneously (which is fine — they are different devices, separately named).

---

Examples

Example 1 — "Swap default output to my headphones"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py list-devices     # find exact name
uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py set-default "Sony WH-1000XM5"

Example 2 — "Capture system audio into OBS"

1. Install BlackHole 2ch:

   uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py blackhole-install --variant 2ch
   # copy the printed brew command and run it
   

2. In Audio MIDI Setup build a Multi-Output Device combining your speakers + BlackHole; set it as system output.

3. Point OBS's "Audio Input Capture" at BlackHole.

Example 3 — "Convert a WAV archive to Apple Lossless"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py convert \
  --input master.wav --output master.m4a --format m4af

Example 4 — "Read EXIF from an AIFF"

afinfo covers it:

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py info recording.aiff

Example 5 — "Speak build results to me"

uv run ${CLAUDE_SKILL_DIR}/scripts/macaudio.py tts "Tests passed, zero failures" --voice Samantha

---

Troubleshooting

SwitchAudioSource: command not found

Cause: Not installed. Fix: brew install switchaudio-osx. The wrapper points at this in its error output.

Default device "silently" reverts after plugging in USB interface

Cause: macOS default-reassign behavior when "When playing sound through" is set to "whatever device is connected". Overrides your scripted default. Fix: System Settings → Sound → Output → uncheck "Automatically switch to newly connected devices" (macOS 14+), or persist the default in a LaunchAgent that re-runs set-default on device change.

afconvert: Error -50 (invalid parameter)

Cause: Requested an output format/rate/bit-depth combo that the codec rejects (e.g. ALAC in WAV container, 32-bit float into M4A). Fix: Drop mismatched flags. ALAC belongs in m4af only; bit-depth is AAC-ignored.

BlackHole doesn't appear after install

Cause: System Extension blocked by macOS. Fix: System Settings → Privacy & Security → scroll to the "System software was blocked" banner → Allow. Reboot. Retry.

afplay works but no sound

Cause: afplay uses the current system default output. If you set a weird output recently (aggregate without speakers as a member), nothing audible is reached. Fix: Re-run set-default with a known-good physical output and retry.

Aggregate Device session drifts

Cause: No drift correction, or drift master isn't the most-stable clock. Fix: In Audio MIDI Setup, check "Drift Correction" on every non-master member. Put the interface with the best clock (Thunderbolt → PCIe card → USB 2) as master.

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Reference docs

• BlackHole vs Loopback vs Background Music comparison, plus the Swift snippet for programmatic Aggregate Device creation → references/virtual-drivers.md.