A new version of the Sound Indicator has been released in Ubuntu 10.10 Maverick Meerkat, with a cool new feature: cache the applications so you can launch them from the Sound Indicator even if they're closed. I tried this and unfortunately Rhythmbox crashes 2 seconds upon closing it and then disappears from the Sound Indicator.
The Ubuntu-mono icon pack also got an update with brand new player icons for the Sound Indicator. But they look a bit... weird:
Also, clicking anywhere to the right of the new player icons, Rhythmbox pauses.
Well, it's still work in progress and the Ubuntu 10.10 Maverick Meerkat final release is 3 months away so there's enough time to improve.