Additional options when setting the music.

We don't need it, but wouldn't it be awesome if we could manipulate tracks, doing more than just setting the start position? I'm thinking:
- Playback speed. If you want a faster-paced theme for when you're under the pressure, or a slower-paced one when you're underwater or underground, this might help.
- Playback volume/pitch. Changing how deep or squeaky the background music goes can sometimes give an alternate theme when done right. Even simply changing its volume can do the same!