Note: You can also get What's Cooler Than Being Cool and Full Medal Panic with this method.
1. Select Co-op Bust a Move
2. Turn your mic volume down to zero
3. Turn your TV volume up
4. Hold the mics to your TV speakers
Took me two tries to get. Don't forget to dance and shake to build star power faster and get the Party and Performance medals.
Note: According to TrueBlade94, this has been patched out but still works if you play offline having cleared your system cache.
The Full Medal Panic! achievement in Lips worth 94 pointsGot 12 medals total in one song (Awarded to all signed in players)