Both Gareth and I have just completed a test using a headset with a microphone attached.
Voice chat is a standard feature In-world which you can set by opening Edit/Preferences. Then select Voice chat/ Enable Voice chat.
We found this was very easy to communicate with each other but limited in the sense that you have to be within a certain distance of each other, just like in real life where the further you move away the quieter your voice becomes to others.
This feature would enable us to play music or do a performance without using applications like Shout Cast. This of course would be limited to a small venue.
The restraints that we found were loss of signal. After running some tests we rectified this with a different headset that seems to have solved the problem.