At first, here's an article from Microsoft (for Vista, Win7 has the same issue):
The processing of network packets on a computer is a resource-intensive task. On high bandwidth links, network traffic consumes even more system resources. Additionally, multimedia programs are time sensitive, and such programs require prioritized access to CPU resources.

Because multimedia programs require more resources, the Windows networking stack implements a throttling mechanism to restrict the processing of non-multimedia network traffic to 10 packets per millisecond.

The throttling will come into effect only when you are running multimedia programs that are time sensitive. However, this throttling mechanism can potentially cause a decrease in network performance during the active multimedia playback period.
Here's the fix (taken from http://forums.steampowered.com/):
* Open regedit
* Go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Multimedia\SystemProfile
* There will be an entry for network performance throttling ("NetworkThrottlingIndex" - zal), default value is 10; set it to FFFFFFFF hex (willl look like 0xFFFFFFFF to the right of the entry)
* Reboot

This removed the lag I was experiencing while still enabling me to have normal sound in-game. Specs in profile.
Enjoy!