Man pages matching name al_set_mixer_postprocess_callback in section 3alleg5:

al_set_mixer_postprocess_callback.3alleg5.en	debian 10 / allegro5-doc
al_set_mixer_postprocess_callback.3alleg5.en	debian 11 / allegro5-doc
al_set_mixer_postprocess_callback.3alleg5.en	debian 12 / allegro5-doc