Oooo, this makes me so mad! The culprit may be PulseAudio - unstable, experimental software that the developers insisted on making the default in Karmic! There is a helpful thread on Ubuntu Forums that might be useful
here. PulseAudio has been a major headache for alot of Ubuntu and Kubuntu users (Xubuntu does not ship with PulseAudio).
It could also be a hardware issue, though. Maybe one of the gurus here can offer some help if the link above doesn't help.
-Robin