void voice_set_playmode(int voice, int playmode);
PLAYMODE_PLAY
Plays the sample a single time. This is the default if you don't set the loop flag.
PLAYMODE_LOOP
Loops repeatedly through the sample, jumping back to the loop start position upon reaching the loop end.
PLAYMODE_FORWARD
Plays the sample from beginning to end. This is the default if you don't set the backward flag.
PLAYMODE_BACKWARD
Reverses the direction of the sample. If you combine this with the loop flag, the sample jumps to the loop end position upon reaching the loop start (ie. you do not need to reverse the loop start and end values when you play the sample in reverse).
PLAYMODE_BIDIR
When used in combination with the loop flag, causes the sample to change direction each time it reaches one of the loop points, so it alternates between playing forwards and in reverse.