OK you all might have not noticed this, but I did.

1. Go to the end of the movie where Josie is singing and then the guy kisses her.

2. As you can see, Val comes up behind them to remind Josie that she's doing a concert, not a makeout session.

3. The slipup - you still hear the bass and drums playing, right? But look at Val when she comes up to break up Josie and her guy. Her arm isn't moving. That means she isn't playing the bass, but there is still music in the background.