ALLEGRO_MIXER - Allegro 5 API

       A  mixer  mixes  together  attached  streams  into  a single buffer.  In the process, it converts channel
       configurations, sample frequencies and audio depths of the attached sample instances  and  audio  streams
       accordingly.  You can control the quality of this conversion using ALLEGRO_MIXER_QUALITY.

       When  going  from  mono  to stereo (and above), the mixer reduces the volume of both channels by sqrt(2).
       When going from stereo (and above) to mono, the mixer reduces the volume of the left and  right  channels
       by sqrt(2) before adding them to the center channel (if present).

Allegro reference manual                                                                  ALLEGRO_MIXER(3alleg5)

       ALLEGRO_MIXER - Allegro 5 API

              #include <allegro5/allegro_audio.h>

              typedef struct ALLEGRO_MIXER ALLEGRO_MIXER;