Tweak bacteria death sound

[hgcummings]

The bacteria death sound is proportional to the number destroyed. This has a nice epic effect when you wipe out hundreds of them, but it's a bit too subtle when you only destroy a few. Given that we also don't play the sequence in this case (to avoid slowing down very skilled players) it's a bit confusing.

From 'goerp' on Ludum Dare:

I would have liked to hear a sound when I destroyed a bacterium.