I'm currently on the third go-round with an 8th grade drama project on our LAN-based sim. As you could imagine the requirements for voice capabilities are pretty specific in a virtual drama project and after trying several options I've finally found the perfect solution--Teamspeak. Teamspeak is easy to set up, allows up to 32 slots on the free version, and uses a cross-platform codec, Speex, so I can have students on Macs and PCs in the same session. Client configuration is easy as well, allowing for settings I pre-configure, like the server IP, to be global, applying to all users, so students don't have to set anything up. All they do is open the client, type their own name in the connect dialog, and since it has the host IP already in the global settings they can ignore that part. I've set up channels for the students to be able to rehearse in groups and not have to hear the other groups, 6 channels in all.
What I tried before is the Opensim's onboard voice module, FreeSWITCH, which is fine but not spatial. That was a deal breaker because students couldn't practice using voice in simultaneous rehearsals. I heard that Vivox was being integrated as a new voice module and is spatial but after looking into that I found that it costs thousands to get a license to host it. Then I tried Ventrilo, often used by World of Warcraft players, and it worked until I got more that 8 students on the server. Only 8 slots unless you rent server hosting! So I ran two Ventrilo servers on two different computers and had to go around connecting students to different IPs, making sure they were with their group partners. Much hassle.
So until Vivox offers an affordable package for use with Opensim Teamspeak will be the best option for this project. Which is a project I should describe in more detail. Will do, soon.