I've also posted this in the Steam forum. When I try to play co-op (always unsuccessfully) and then exit the game, the Windows system sounds are broken (wrongly played). For example, check the clicking sound for navigation in Windows or Internet Explorer: It sounds very faded. If I restart the computer the sounds get back to normal. Also, trying the single player mode does not break sound at all. Definetely not a driver issue, I'm on latest drivers, DirectX etc. Running on a Windows 7 Ultimate 64bit.
Thanks in advance for your help.