I would try to deactivate the exclusive mode. Get into your Windows sound settings, on the recording tab, open the properties of your microphone, switch to the last tab "Advanced" and deactivate both checkboxes, apply and/or click ok and restart the PC to make sure everything works and nothing still has exclusive mode. Then try again with obs and mumble.
Since I havent heard of such a problem yet, I think that it should be fixed with that.