I just installed Ubuntu 9.04 and have been unable to get sound functioning. I have a set of USB speakers (not USB powered with a stereo plug... they are totally USB).
I've tried plugging them directly into the tower as well as into the USB port on my Dell 2405FPW monitor. Both USB ports are functioning correctly (I tested by sticking a flash drive in there - they both read it), and the speakers are functioning correctly in Windows.
If it's relevant, I have an SB Audigy 2 sound card that came with the computer, but is not being used.
EDIT: These are the speakers - Logitech S-150 USB Speakers