The ability to speak (voice) is now part of the main Second Life viewer per “The Second Life Voice Viewer is Live!” a 2 August 2007 post in the Second Life Blog. 45,000 plus viewers have downloaded and tested it.
Voice is an optional feature and is turned off by default. It is available everywhere, land owners can control how it is used on their property.
The code is open source. The entire voice system is hosted on separate servers to not impair performance of the normal Second Life experience.