This problem has been intermittent - more than once it has just quit for no apparent reason and not re-appeared for months at a time.

I haven't seen any relation to updates or newly installed programs, it has been coming and going for some time now with various kernels.

This installation was originally Natty onto which I have added the xubuntu-desktop. The behavior has occurred both before and after that change.

I will start an audio app playing local mp3's (or flacs/OGGs or whatever in VLC or XMMS - haven't tried it with other players) or streaming audio (and audio/video such as youtube) and it will play for a few minutes and suddenly there is a buzzing noise, as if the music freezes and keeps repeating a note. It sounds almost electrical, similar to 60 cycle humming but a much higher frequency, depending on the music being played.

On occasion, but not always, this will occur if I click a link on a web site. That may just be co-incidental.
Other times videos will seemingly play with no problem, though I have never continued listening/watching videos until it does happen - eventually it probably would.

If I stop VLC or XMMS and restart them two or three times it will start the track over and again play for a minutes or three.

Hitting the pause button repeatedly on either, sometimes gets sound again and sometimes not.

Using XMMS I can cause the problem by advancing the track, then advancing again or backing up to an earlier spot will resume playback.
With VLC advancing/rewinding has no bad effect - at least so far in my testing.

In addition, with both players, sometimes upon first opening and trying to play a track I see the progress bar moving - and with XMMS the equalizer display is looking like the track is playing - but there's no sound. Hitting stop then play or the pause button repeatedly will eventually start playing the track (with sound).

Also, again - sometimes - when automatically advancing to the next track there will be no sound unless I pause/play repeatedly or stop/start the track again. (Both players)

On hitting the stop button after it's started getting noisy, the noise will sometimes continue and VLC's and XMMS's progress bars stop.
Sometimes hitting either one's stop button when playing a track triggers the noise.

Running either from a terminal shows no error when the noise happens.

If it's something streaming, opening VLC or XMMS and playing a track will stop the noise and the track will play for a bit. If I pause the stream and restart it will not correct the problem, though re-loading the pages does.

I've tried two identical sound cards with the same result (CS46xx), though I haven't tried the onboard sound card (Intel, I believe), both sound cards work in other machines with no error.
I get the feeling it's software related.

Not sure how I'd continue troubleshooting this but I am going to start over, once again <sigh> with this:
http://ubuntuforums.org/showthread.php?t=1885240

So far search results haven't turned up anything very helpful(seems no one else has experienced the same problem)though I've tried more things than I can remember.
If you have any ideas I'd appreciate hearing them.

Incidentally after a recent re-boot the volume controls/mute keys on my keyboard suddenly have no effect. I seem to remember there's a setting to enable this and they have been working for many months until just the other day but I can't seem to find it.