HelenOS sources
AUDIO_CAP_PLAYBACK 354 uspace/app/wavplay/dplay.c ret = audio_pcm_query_cap(session, AUDIO_CAP_PLAYBACK, &val);
AUDIO_CAP_PLAYBACK 106 uspace/drv/audio/hdaudio/pcm_iface.c case AUDIO_CAP_PLAYBACK:
AUDIO_CAP_PLAYBACK 279 uspace/drv/audio/sb16/dsp.c case AUDIO_CAP_PLAYBACK:
AUDIO_CAP_PLAYBACK 69 uspace/lib/drv/generic/remote_audio_pcm.c [AUDIO_CAP_PLAYBACK] = "PLAYBACK",
AUDIO_CAP_PLAYBACK 140 uspace/srv/audio/hound/audio_device.c errno_t rc = audio_pcm_query_cap(dev->sess, AUDIO_CAP_PLAYBACK, &val);
HelenOS homepage, sources at GitHub