About your sound issues in battlescape -
Does it only happen on battlescape?
Does it happen on Geospace too?
What if you close the game and start playing a song on Winamp for example?
I've noticed on my system that high CPU usage, caused by a lot of
DPCs, is causing "static" in my sound.
A restart of the problematic program immediately results in a drop of DPC-related CPU usage, until it builds up again (as the program accumulates DPCs again).
My "problematic program" is uTorrent, BTW.
If this sounds remotely like the problem you're having, I can tell you how to determine DPC-related CPU usage, if you're on Windows.