Minecraft sound with Pulseaudio

When using PA, I would get no or choppy sound. Switching to padsp rather than aoss fixes it.

I was careful not to change the default pulseaudio derivation as not to trigger Chromium rebuilds.

It would be better if we could get the Minecraft OpenAL to work with PA, but Minecraft supplies its own OpenAL, which we might have to somehow prevent it from doing. Running in padsp is just much easier. :)