Music/sound effect sequence playback

[freem]

Part of the Sound Driver master todo list (#2)

The real core of the sound driver is being able to play back sound effects and music. This is the part of the sound driver that will take the longest to complete, considering it's the most important element.

(this space reserved for current thoughts; please see docs/functions/sound.txt and docs/functions/sound/ directory for what's been written down)