I'm experiencing this problem with Ubuntu 8.10 Intrepid 32 bit, new install. MSI EX600 laptop, Intel T7100 processor, 2G DDR2 667 MHz memory, Nvidia GeForce 8400M GS video adapter. Compiz is turned off, the sound preferences are set to ALSA.
It shows with rhythmbox and totem (gstreamer), while scrolling in firefox 3.0.4 or nautilus file manager. xorg cpu usage peaks up at the moment the sound lags. When visual effects in totem are turned off, the problem doesn't show itself.
With audacious, audacity, vlc, realplayer and smplayer there is no problem.
xorg version is: