i had this problem, well occasionaly it happens..

1. are you using a laptop woth a cam/mic?
(if so. unplug and plug back in your mic occasionaly tell it registers and works in steam)

2. if not.. right click on your volume control in your task bar then click the "play back devices" tab, click on the enhacments tab and make sure no enhancments are turned on. (if non are turned on, do you have the proper software for your mic ou are using? )

3. to make sure... press ctrl + Shift + Escape. Look for a program named PNKSTB (it starts with PNK) and cancel that.. (it is a program that speeds up media downloads from microsoft) cancel it, because it effects your sound (so i experienced)

4. if none of this works.. i have no idea. make a fat chick your desktop background??