I had the same problem, I have Win7 64Bit 12G ram I7 975,MSI Eclipse plus with Sound Blaster X-FI Xtreme MSI EDITION 5870 CrossFireX2 also using Bose Series 5 Speakers on usb2, All my games run great. Was using Steam Trainer DX9-11 version but ran game on DX11 and Iwould get sound on the startup menu but would lose audio on main game. Ifollowed SUPERSIZE's advice and ran game on DX9 from menu and I have no problem so try running on DX9.
try turning off mouse acceleration and make sure youre sound settings for your computer are set to 44800khz