Here are a few fixes that can help cure the distortion problems Mumble can have
1. Open Mumble.
2. At the top of the Mumble program click the "Configure" option then click "Settings".
3. At the bottom left of the program select the "Advanced" option.
4. Try lowering the codec quality down a few selections, as well as increasing the audio per packet setting up a few selections.
5. Try increasing the Jitter Buffer up a few selections and see if this helps.