Just a guess but I'm thinking maybe an IRQ conflict. Check Windows Device Manager for yellow exclamation points or red X's.
I'd check the function through Windows Control Panel, Sound Speech Audio Devices, Sounds and Audio Devices, Voice tab, Hardware test button.
If that goes ok, check the advanced buttons to make sure things are set proper.
If everything goes ok, check in game voice setup to see if anything is wrong or different there.
As a last resort, I'd check for new driver for your sound card or maybe even roll back the driver if you've updated lately and are having problems now.
Best I can do.
Good luck.