al_create_voice man page
al_create_voice — Allegro 5 API
#include <allegro5/allegro_audio.h> ALLEGRO_VOICE *al_create_voice(unsigned int freq, ALLEGRO_AUDIO_DEPTH depth, ALLEGRO_CHANNEL_CONF chan_conf)
Creates a voice structure and allocates a voice from the digital sound driver. The passed frequency, sample format and channel configuration are used as a hint to what kind of data will be sent to the voice. However, the underlying sound driver is free to use non-matching values. For example, it may be the native format of the sound hardware.
If a mixer is attached to the voice, the mixer will handle the conversion of all its input streams to the voice format and care does not have to be taken for this. However if you access the voice directly, make sure to not rely on the parameters passed to this function, but instead query the returned voice for the actual settings.