Ubuntu needs new sounds, and with the beginning of the transition to the new theme and the new boot experience, Karmic would be a great time to implement it.
What I'm proposing is a complete overhaul of the sound set, with the emphasis being on cues when something is actually done. For example, instead of a sound when a question dialog appears, it would be better to have a sound which was played when the Trash was emptied.
Shorter, simpler sounds should be the focus: instead of the melodic sounds that have traditionally accompanied Ubuntu (and Windows) in the past, the sounds should be in the vein of the sounds Mac OS X uses. For example, the Macintosh's boot chime
(the equivalent of the BIOS boot beep - nothing to do with Ubuntu, but still important as a reference) is a single chord.
So instead of the current set, sounds should perhaps be like this: on boot, a short jingle or chord. When an item is dragged to the trash, it should sound like something being screwed up or thrown into a bin. When the trash is being emptied, it should sound like an empty trash. On e-mail coming in, it should sound like the flap of a letterbox and an envelope falling on to the floor. On an incoming instant message or IRC message (a sound which is regular), a quiet sound like the tapping of a pen on a desk, or a simple, quiet beep (like Colloquy on Mac OS X.)