If you have more than one sound source, another mic plugged in, even if its embedded in another device like a webcam, make sure they are all disabled , and your headset mic is set to the primary audio device.
A bad cable on a set of headphones I had meant that windows would switch the audio device on me, mid game. It used to be you could switch it back using the audio control panel and make sure the right source was set in TS options, if you are in game now though it may not work and it might require you to restart the game, after re-selecting the audio source.
Again, check TS options to make sure your current audio capture device is your preferred device., if it isn't, use the windows audio control panel to set it back, and then do the same in TS options for the capture device.