I'm trying to use Hydrogen drum machine, but the sound only works for the metronome. I find this odd, because sound has worked for all other programs, and it worked perfectly with 7.04 (I never tried with 7.10).

Sound in 8.04 is a bit funky to say the least. Pulse Audio is the source of most problems and I haven't decided if I intend to revert back to ALSA or not. Pulse has a little ways to go and not everything works well with it.

Some programs (Hydrogen apparently also) don't play well with Pulse. I believe that if you ensure all other sound device programs are closed, then it will free up ALSA for it,.... BUT, these kinds of devices usually take complete control of it and prevent sounds from other apps until it is closed.

Search for "perfect Pulse Audio Configuration." I think Pulse is likely your problem.