Anyway, right after installing hydrogen with apt-get, I tried to launch it. It wouldn't get past the splash screen due to a problem with the jack library packages that causes a fatal error on ARM based systems. If I specified that I wanted to use ALSA directly ($/> hydrogen -d alsa), then everything worked fine, so hydrogen itself seems to be OK.
The fix for the "FATAL: cannot locate cpu MHz in /proc/cpuinfo" problem on ARM is available in the latest jack2 source code, so I pulled it from github, compiled, and installed. I was able to configure and start the server via qjackctl with no real issues other than a warnings that my system doesn't have MIDI sequencer (which doesn't matter to me.)
Then I relaunched hydrogen and let it connect to jack. Everything seemed OK on the surface. Hydrogen showed up with proper connections in the qjackctl window, and I thought for sure I was done. HOWEVER, when I pressed play to listen to one of the hydrogen demo songs it just about blew my ears out. I got some painful screeching/hissing and had to rip off the headphones.
After some messing around, I discovered that pulling the master fader on the hydrogen mixer down VERY low caused that horrible noise to start to resemble drums. Pulling it to the lowest possible value above zero, and it was almost tolerable to listen to. The sound coming out of the headphones is not loud with the mixer down this low, but the signal sounds as if it has been distorted and clipped along the way.
So, what am I missing here? There seems to be some massive (over)amplification going on in the layers between hydrogen and my soundcard that only shows up when jack is involved. Hydrogen going straight to ALSA works fine.