Since you're trying to load a raw, filesystem-based file (as opposed to an imported resource), I think you'll have to work a little harder. Something like this should work (for example):
var path = "/path/to/some.mp3"
var snd_file=File.new()
snd_file.open(path, File.READ)
var stream = AudioStreamMP3.new()
stream.data = snd_file.get_buffer(snd_file.get_len())
snd_file.close()
$AudioStreamPlayer.stream = stream
$AudioStreamPlayer.play()