qmk-firmware/quantum/process_keycode/process_audio.c

#include "process_audio.h"
#include "audio.h"

static float compute_freq_for_midi_note(uint8_t note)
{
    // https://en.wikipedia.org/wiki/MIDI_tuning_standard
    return pow(2.0, (note - 69) / 12.0) * 440.0f;
}

bool process_audio(uint16_t keycode, keyrecord_t *record) {

    if (keycode == AU_ON && record->event.pressed) {
      audio_on();
      return false;
    }

    if (keycode == AU_OFF && record->event.pressed) {
      audio_off();
      return false;
    }

    if (keycode == AU_TOG && record->event.pressed) {
        if (is_audio_on())
        {
            audio_off();
        }
        else
        {
            audio_on();
        }
        return false;
    }

    if (keycode == MUV_IN && record->event.pressed) {
        voice_iterate();
        music_scale_user();
        return false;
    }

    if (keycode == MUV_DE && record->event.pressed) {
        voice_deiterate();
        music_scale_user();
        return false;
    }

    return true
}

void process_audio_noteon(uint8_t note) {
    play_note(compute_freq_for_midi_note(note), 0xF);
}

void process_audio_noteoff(uint8_t note) {
    stop_note(compute_freq_for_midi_note(note));
}

void process_audio_stop_all_notes(void) {
    stop_all_notes();
}

__attribute__ ((weak))
void audio_on_user() {}