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
A special value ALLEGRO_AUDIO_PAN_NONE(3) 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[aq]t work yet.
Returns true on success, false on failure.
Will fail if the sample instance is attached directly to a voice.