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The built-in sink uses CPAL to play audio through your operating system’s default audio device. This is the default and needs no setup beyond installing Rockbox. 
music_dir    = "/path/to/Music"
audio_output = "builtin"

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Selecting the output device

CPAL plays through whichever device the OS reports as default. To change it, use your platform’s audio settings:
  • macOS — System Settings › Sound › Output
  • Linux (PipeWire / PulseAudio) — pavucontrol or your DE’s sound applet
  • Windows — Sound settings › Output
There is no per-application device picker built into Rockbox for the CPAL sink. If you need multi-device routing on Linux, point Rockbox at a PulseAudio null sink and route from there.

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macOS-specific notes

CPAL uses CoreAudio natively on macOS — no additional setup is required. If you ever see “no audio output” on macOS only, verify that the correct output device is selected in System Settings › Sound › Output.

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Format

Output is S16LE stereo at 44 100 Hz. Higher-resolution sources are dithered down by the rbcodec DSP pipeline before they reach CPAL. To change the output sample rate, set play_frequency in settings.toml (auto, 44100, 48000, 88200, 96000).

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Switching to another sink

Edit audio_output in settings.toml and rockbox restart, or call: 
mutation { connectDevice(id: "<device-id>") }
…with a device discovered via mDNS — see Audio output › Overview.