I have a similar problem with my Windows 7 b7100 in FF and IE8
With some sites I get about 1/2 second of regular sound then I get a low tone thumping, beating sound. Ive reinstalled FF and Flash, no change. If I reboot, the site I did not sound with may work, but other boots I get the bad sound from the same site. Games, and music from Itunes sound fine.
I've only had this happen with build 7100 x64, all older builds things worked fine.
(ps build 7100, seems, to me, to run slower, load slower, run programs slower than previous builds from fresh installs on the same machine, fyi)