Design decision. I figured more people would just want to have no music for the extra concentration on the harder levels, or just play their own music. Instead of having music for each level, we just made added support for playing your own music library. The shotgun usually helps. The lowest level of the shotgun is a good 5 second stun if the Chaser is about to eat you. If you haven't done monkey yet, try that instead, as it's a little easier than bat. Other than that, we encourage trying to get better scores on lower levels if you get stuck, since beating Developer Scores gives you a lot of money for upgrades.