Are you saying that there might not be anything wrong with the app other than Require a MIDI app to be installed as well? But doesn't ALSA have a MIDI driver? At least that is what is on the Hydrogen prefs and the drum machine is running great.
Yes, but the midi app has to be running, not just installed. For example, 'timidity -iA' will run timidity as a server. Then many midi programs require you to select your midi server/connection. Only then do you get sound. I haven't used it in years, but hydrogen always worked because it provided it's own midi capabilities, just like mscore, improvisor, tuxguitar and I think rosegarden, also.