I know Razer headsets are affected by this issue:
http://voiceattack.com/SMF/index.php?topic=152.0However, that one will flat-out prevent your audio from getting through (wouldn't hurt to set it, however).
If you can't get it working with a driver update, and it doesn't seem to be a wiring issue, and you don't have some other type of software that is muting the input channel, you may need to try another headset. It's a passive monitor that just checks the state of the speech engine. If the speech engine says, 'Hey, the volume is muted on the input channel', VoiceAttack displays that icon.