audio_playback
[AUDIO_PLAYBACK_INDEX] = &audio_playback,
usb_make_str_desc(&audio_playback, sizeof(audio_playback),
&audio_playback, sizeof(audio_playback), usb_temp_sysctl,
static struct usb_string_descriptor audio_playback;
int audio_playback(struct audio *aud, const uint8_t *buf, size_t count);
err = audio_playback(aud, st->buf, sizeof(st->buf));