The bug:
Unity doesn't reroute the sound as audio input and output change

Repro steps:
1. Open the project I attached and select the audio clip in it
2. Start playing the track in the Inspector
3. Plug in a usb headset - the music still goes on in the main speaker
4. Stop the music in the Inspector and press Play again - the music plays now in the headset properly
5. While the music plays, unplug the headset - both the music and track's time indicator stop
6. Stop and play track again - the music plays correctly in the headset.

Note:
- The error: 'Invalid device usage' shows when the headset is plugged in
- reproduced on Mac 10.7.4, Mac 10.8.1 and Windows 7
- Laurent claims he has to restart the editor, but I didn't manage to repro that