Hey, just to let you know it's possible to run into a rare crash if the audio is muted and there is "No song playing", and you then hit the "previous song" button.
I fixed it by adding the same code from the next() function to the previous() function, right before "pos = renpy.music.get_pos(channel=self.channel)":
filename = renpy.music.get_playing(channel=self.channel) if filename is None: return self.play(None, 0)