al_set_default_mixer - Allegro 5 API

       Sets  the default mixer.  All samples started with al_play_sample(3alleg5) will be stopped and all sample
       instances returned by al_lock_sample_id(3alleg5) will be invalidated.  If you are using your  own  mixer,
       this should be called before al_reserve_samples(3alleg5).

       Returns true on success, false on error.

       al_set_default_mixer - Allegro 5 API

al_reserve_samples(3alleg5),            al_play_sample(3alleg5),           al_get_default_mixer(3alleg5),
       al_restore_default_mixer(3alleg5)

Allegro reference manual                                                           al_set_default_mixer(3alleg5)

              #include <allegro5/allegro_audio.h>

              bool al_set_default_mixer(ALLEGRO_MIXER *mixer)