Change background music:
Press the Left Difficulty switch.

Hint: Completing level 2-3:
There is a large gap in level 2-3 that Louie must cross. Jump on the middle sewer pipe, then jump on the pipe to the left. Walk to the left edge of this pipe and, then run to the right. Continue running until Louie reaches the edge of the third, and lowest, pipe in the series. Then, jump to cross over the sludge gap.