Check your firewall. I have had a similar problem. I went to the firewall settings, where all exceptions are listed that do NOT get blocked and deleted all entries with the name SL voice. After starting SL again, my firewall asked me to confirm if SL voice is allowed or should be blocked. I allowed it and then it worked again. I use Windows Vista.
I also have problems with streaming music. I have evyrthing switched on, checked all settings - but no music. Strangely I can hear the sound that comes with a movie file, I hear all silly gestures, birds, fire cracking etc. But no music. Kind of silly if you go to a party. ..