Mumble uses Opus by default. You can definitely tell the audio sounds much better than competitors although mumble is more fussy about mic and background noise as it does less processing than Skype for example.

Opus was originally intended to be a general purpose codec that maintained fidelity at even low bitrates which makes it suitable for telephony as it's also low latency. On the other hand it also sounds good at higher bit rates and I use it as my codec for my music when I convert them from flac for mobile devices.