I haven’t been successful in getting this to work with the java sound either. The only way that I could get the sound to work is to first install “Timidity” from the repositories. Once this is done you will need to enter the following command into a konsole. This has to be done before you start Tuxguitar.
timidity -iA -Os
This starts timidity running. You can then close the konsole and open TuxGuitar and select the “Timidity port 0 [128:0]” under the Midi port settings.