al_set_sample_instance_gain - Allegro 5 API

       Set the playback gain of the sample instance.

       Returns  true  on  success, false on failure.  Will fail if the sample instance is attached directly to a
       voice.

       al_set_sample_instance_gain - Allegro 5 API

al_get_sample_instance_gain(3alleg5)

Allegro reference manual                                                    al_set_sample_instance_gain(3alleg5)

              #include <allegro5/allegro_audio.h>

              bool al_set_sample_instance_gain(ALLEGRO_SAMPLE_INSTANCE *spl, float val)