bool al_set_sample_instance_pan(ALLEGRO_SAMPLE_INSTANCE *spl, float val)
Set the pan value on a sample instance. A value of -1.0 means to play the sample 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 sample at its original level. This will be louder than a pan value of 0.0.
Note: panning samples with more than two channels doesn't work yet.
Returns true on success, false on failure. Will fail if the sample instance is attached directly to a voice.