al_set_audio_stream_pan - Allegro 5 API

       Set  the  pan  value  on an audio stream.  A value of -1.0 means to play the stream only through the left
       speaker; +1.0 means only through the right speaker; 0.0 means the sample is centre balanced.   A  special
       value  ALLEGRO_AUDIO_PAN_NONE(3alleg5) disables panning and plays the stream at its original level.  This
       will be louder than a pan value of 0.0.

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

       al_set_audio_stream_pan - Allegro 5 API

al_get_audio_stream_pan(3alleg5), ALLEGRO_AUDIO_PAN_NONE(3alleg5)

Allegro reference manual                                                        al_set_audio_stream_pan(3alleg5)

              #include <allegro5/allegro_audio.h>

              bool al_set_audio_stream_pan(ALLEGRO_AUDIO_STREAM *stream, float val)