does the sound go as well as the screen? If you haven't noticed,
could you try playing some music and see if you can still hear it when
the screen goes blank and let us know?

Also, I can't remember if you've mentioned whether or not this is a
laptop. It may not be relevant but...

Also, could you tell us what video card and driver you have (lspci -vv
[that's two 'vee's not a 'double-you', just in case it's as unclear on
your monitor as it is on mine :-)] or sudo lshw in a terminal)?