OptionalspriteName of a region declared in the clip's sprite map: the track loops on
that region instead of the whole clip. A region of a loop's exact length
keeps it gapless where a decoder pads the end of the clip (Safari, on
MP3).
OptionalvolumePlayback volume, 0.0..1.0. Defaults to the current global volume.
Options accepted by
audio.playTrackin place of itsvolumeargument.