Some time ago there was a thread that mentioned voice-chat as a must-have for future releases.
I did a little bit searching for OS vchat software. What I found are those three:
http://mumble.sourceforge.net/
Perhaps it would be easier for devs to integrate such a system directly into nexuiz than coding one on their own.
Otherwise: Serveradmins might set up servers for "roger wilco" or "teamspeak" - as long as their servers are not used commercially
or we could all use Mumble, it has a dedicated server named Murmur
Found some clients for XFire (using GAIM) and RogerWilco:
http://linux-wildo.sourceforge.net/
http://www.fryx.ch/xfire/