Howard Nurse, W6HN
Here are some things to check.
1. Check that the audio settings are correct on the Raspberry Pi desktop by right-clicking the Speaker icon.
2. Check that the audio settings are correct in Mumble>Configure>Settings>Audio In and Audio Out.
3. Are the lips red for the Pi user and your phone user?
4. If not, make sure Mumble in both places is set to Transmission>Continuous.
5. In Mumble on the RPi desktop, go to Configure>Settings>Audio in and temporarily change Transmission to Audio Level, then click Applu.
6. Dose the bar graph bounce around showing radio noise?