I’m using a MacBook 10.6.8 and Audacity 2.0.6. I can (I think) record, but I cannot play back. I can easily play anything on Itunes, but nothing happens with Audacity.
Unplug your USB sound devices and restart Audacity. It should default to your internal speakers.
Also read where Audacity is playing to from the Device Toolbar settings.
Audacity Recording and Playback can be independent of other players and software. That’s what gives you the ability to play tricks like overdubbing.