Normally it shouldn't stay muted, but when the Windows default multimedia recording device changes (which would likely happen when you start using your VR headset, E.G. by running Steam VR), VoiceAttack mutes both the device that was the default, and the device that has become the default, momentarily, before changing the device the speech recognition system is listening to.
Muting the previous default device is necessary due to a quirk of the speech recognition system, where if it's still performing recognition (which can occur as long as there is input, including line or background noise), it cannot be shut down.
However, this is only supposed to last for a few seconds, after which both devices are instructed to unmute again.
Another user also reported having their G2 VR headset, specifically, remaining muted, though in their case it appeared the latest VoiceAttack version no longer exhibited that behavior.
It may be that the manner in which the headset is initialized causes the unmute instruction to (perhaps sporadically) get ignored somehow
I've passed this along to the developer.