I recently built a PC and I started playing Battlefield 3 with my headset in my PC instead of my xbox, but the sound is audibly different and worse on PC than the Xbox, I'm not sure what it is, the only difference between the set up is on the xbox the connection to the mixer to system is an optical cable and on PC I use a USB, I would have thought the sound would be exactly the same.
Is it the sound card?
I'll list some basic specs which might be the problem:
HEADSET: Tritton AX Pro
GPU: 2GB MSI GTX 660 Ti Power Edition
CPU: Intel Core i5 3570k
MOBO: MSI Z77A-G45
I looked up your Motherboard and if you are using the Audio that is built in with the MOBO it is 8 channels, which is 5.1 surround, meaning your sound is fine. I also have 8 channel sound and it sounds great. Maybe its the headset you have, make sure you are plugging the headset in on the back of the computer.
Different audio tracks? I'd bet on the 360 your getting a Dolby soundtrack, and on the PC you're getting a standard 2.0 audio stream. Plus, you go through a layer of driver software which would affect the final output. Headset might also be converting the differnet audio tracks to analog slightly differently...
I also note that all Tritton headsets are crap quality wise.