Sound test:
Press Up/Right and press Start at the title screen.

Hint: Batarangs:
Always collect Batarangs. They kill enemies quickly from a safe distance.

Hint: Hidden power-ups in level 4-2:
Break the rocks to get power-ups such as the small hearts.

Hint: Defeating Jack and the Joker:
Hit Jack and the Joker with Batarangs. Throw the Batarangs so that they hit Jack or the Joker just as they slow down or stop moving.