When I start up mumble, all my audio cuts out. I then go to my sound & then click on test on my speakers & it tells me "the device is being used by another application". Does anyone know why this is happening?
You most likely enabled "exclusive" mode for the audio device in the mumble settings. As the name says this provides mumble with exclusive access to the device which cuts out some latency in the audio system. Disabling it will allow other applications to share the device with mumble.