I had that problem too. It was a digital mic that connected by USB. Steam uses the default Mic connection. For example on my computer Steam was using the Mic port for the voice, but because my Mic wasn't plugged into the mic port but rather the USB I had to switch the default in which Winows uses to get it to work in Steam. I will try to write a step by step for you because what I just typed probaly makes no sense. It wasn't typed very well.....
