Comment 37 for bug 496616

While there are several different scenarios here, from all my machines that become affected the culprit seems to be the gstreamer0.10-pusleaudio package.

Those affected and still bothering to ck. this bug should see if removing helps.

At least here it only occurs on setups with 5.1 sound and pulse set to output 5.1 analog.
The initial format to cause this would be 5.1 .ac3, whether on a dvd or an audio file.

The same behaviour is continuing into Maverick (10.10

bug report concerning 5.1 .ac3 in gstreamer, both lucid and maverick (as ignored as this bug which is par for the course when concerning pulseaudio
https://bugs.launchpad.net/ubuntu/+source/gst-plugins-good0.10/+bug/554002