al_set_default_voice - Allegro 5 API

       You can call this before calling al_restore_default_mixer to provide the voice which should be used.  Any
       previous voice will be destroyed.  You can also pass NULL to destroy the current default voice.

       al_set_default_voice - Allegro 5 API

al_get_default_mixer(3alleg5)

Allegro reference manual                                                           al_set_default_voice(3alleg5)

       5.1.13

              #include <allegro5/allegro_audio.h>

              void al_set_default_voice(ALLEGRO_VOICE *voice)