al_create_mixer man page
al_create_mixer — Allegro 5 API
#include <allegro5/allegro_audio.h> ALLEGRO_MIXER *al_create_mixer(unsigned int freq, ALLEGRO_AUDIO_DEPTH depth, ALLEGRO_CHANNEL_CONF chan_conf)
Creates a mixer to attach sample instances, audio streams, or other mixers to. It will mix into a buffer at the requested frequency and channel count.
The only supported audio depths are ALLEGRO_AUDIO_DEPTH_FLOAT32 and ALLEGRO_AUDIO_DEPTH_INT16 (not yet complete).
To actually produce any output, the mixer will have to be attached to a voice.
Returns true on success, false on error.
al_destroy_mixer(3), ALLEGRO_AUDIO_DEPTH(3), ALLEGRO_CHANNEL_CONF(3)