Use Your Gamer Launch Account
Your account from any Gamer Launch site is valid here.
If you already have a Gamer Launch account connect your Facebook account on your user profile first before using Facebook connect to login. Otherwise you may end up with a new account.
Next, under Control Panel --> Sound, go to the Recording tab, click on your active microphone and click Properties. Click on the Level tab and make sure your microphone level is turned up. It shouldn't be turned WAY up, but if it was turned down, then it is hard for Mumble to compensate for that. Also, you can go into the Configure and run the Windows microphone audio Wizard (different from Mumble Audio Wizard) to try and normalize the volume there.