Implement voice chat using Tox
There's a protocol for voice communication called Tox. It's fully peer-to-peer, end-to-end encrypted DHT-powered communication service, it doesn't require any additional servers. All you need to communicate over Tox is just one C library: https://github.com/TokTok/c-toxcore By implementing this library into Xonotic we automatically get voice chat and contact list (easier to get together and play with friends), as well as some other interesting features that can be implemented using this library.
I remember that there was a controversy around this protocol, and I stopped using it. I'm not sure if one of the popular clients was said to contain malware functionality built-in, or if one of the devs stole the project team's funds and sold the user data, IDK, something nasty.
So I'd advise a thorough due diligence before using Tox protocol, to ensure it's clean and safe. It probably is.