al_set_mixer_postprocess_callback man page
al_set_mixer_postprocess_callback — Allegro 5 API
#include <allegro5/allegro_audio.h> bool al_set_mixer_postprocess_callback(ALLEGRO_MIXER *mixer, void (*pp_callback)(void *buf, unsigned int samples, void *data), void *pp_callback_userdata)
Sets a post-processing filter function that's called after the attached streams have been mixed. The buffer's format will be whatever the mixer was created with. The sample count and user-data pointer is also passed.