I use USB output for sound. Since updating to 11.04 I have witnessed the following problem, which many other users, as described in Bug #761312, also experience:
For a while I can watch a flash video or listen to music from Banshee perfectly fine. In a seemingly random moment the sound starts getting jittery - i.e. a hissing background noise / white noise appears - getting constantly worse, until being totally unusable. When the sound goes bad, it does so across both Flash and Banshee. As many others have described it does not matter was player you are using and it is not gstreamer specific either, as it also occurs with xine etc.
I have tried killing the pulseaudio process to no help. I pause whatever is playing and after some time the sound starts working again.
Strangely, when the problem occurs and I set my sound output to mono, the problem goes away for a short time, but then starts to jitter again fairly quickly.
I have tried Google with no luck and I have not changed any sound settings since upgrading from 10.10
The problem in still present in Oneiric.