Proximity chat won't work
FiveM Voice Chat in game not working!
How To Fix GTA 5 Voice Chat Not Working On PC
Before you go into the settings, make sure that you double check if the microphone plug is connected to the port of your PC.
If you connect your headset with jacks to your sound card, Speakers and Headset might always output the same.
Afaik Mumble does not ship with one by default.