al_set_audio_stream_gain - Allegro 5 API

       Set the playback gain of the stream.

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

       al_set_audio_stream_gain - Allegro 5 API

al_get_audio_stream_gain(3alleg5).

Allegro reference manual                                                       al_set_audio_stream_gain(3alleg5)

              #include <allegro5/allegro_audio.h>

              bool al_set_audio_stream_gain(ALLEGRO_AUDIO_STREAM *stream, float val)