Second Life Voice - Public Beta !!!

How It Works In-World

When you are on a voice-enabled parcel, you can simply walk up to
another avatar and begin speaking in a normal voice (no need to shout!).

If your headset is correctly configured, you will see a voice
"intensity indicator" over your avatar. This indicates that your
voice is being sent to (and can be heard by) others on the parcel.
The voice intensity indicator will change color slightly and will
grow and shrink along with the natural volume patterns and
fluctuations of your voice.

If the indicator turns red, you're either speaking too loudly, the
microphone is too close to your mouth, or the volume control on your
microphone is turned up a little too far. Please make adjustments
until the voice indicator stays in the bright green range and only
seldom flickers to red.

When others speak, you will see their intensity indicators and hear
their voices as they will hear yours. Walk around someone who is
speaking to you to hear the voice move around in 3D, based on where
you are relative to that avatar. If you turn toward someone and move
closer, for example, their voice will be louder. If you speak while
walking around someone else, they too will hear your voice tracking
your position.

All the information and to download:
http://secondlife.com/community/bhear.php