On bluetooth, the microphone just produces choppy output. Tried 3 different brands of bluetooth headsets, and earbuds.. all the same.
Various settings in the preferences don't seem to clear this up. Interestingly, the plumble free app on android had the same problem with the same hardware UNTIL... I went into preferences and it has a half duplex mode that youu can set for push to talk, and voila... clear as a bell.. same bluetooth hardware. Assuming the same codebase is used as a starting point for both, I would think this would be achievable for Mumble as well? Looks like the audio upstream and downstream on the bluetooth connection are trying to share and colliding.

It's not mission critical, but would be nice to use my micro earbud that looks like a little hearing aid, instead of a boom mic headset, or wired earbuds where the inline mic changes orientation and rubs on clothing etc.