The 'Whisper' voice system for OpenSim has been released as open source and is available right now. Whisper is a tight integration of the popular Mumble voice-over-IP code with OpenSim for use with most OpenSim viewers. I've been using Whisper for some time now and wrote about it here - it provides far and away the best voice solution available for OpenSim with full support for lip sync and speaker indication, so having this now fully available to the community should really hasten the adoption of OpenSim as a virtual world platform for serious uses such as education, meetings, collaboration and training. Full details of how where to find the source and how to build both client and server elements are available on the Whisper forums