I just accidentally uninstalled TuxGuitar (it uses Java, which I have a habit of deleting from my system :). Unfortunately, that meant I not only had to reinstall it, but also had to re-find-out how to get sound working. The problem: default midi for TuxGuitar is /dev/sequence, which, by the way, does not exist.
Oookay…
After a search, I found one way to do it:
sudo apt-get install timidity tuxguitar-jsa
This installs a real-time sequencer.
Tweak the settings in TuxGuitar (F7 -> sound) to use this, on the appropriate MIDI port — in my case, Gervill.