I would be surprised if anyone here has extensive experience with audio over Tor. I don't, primarily because I'm not too keen on random anons, and possibly LE, being able to hear and record my voice. However, the Tails people have spoken positively about Mumble: http://mumble.sourceforge.net I've also heard good things about SFLphone: http://sflphone.org Although that's only available for Linux. Presumably for any of these solutions, you would have to run your own server as a hidden service for the client software to connect to. Briefly looking at the Hidden Wikis, I don't see any existing VOIP hidden services.