I am currently writing an applet where you can train your ears to hear intervals, chords etc.
Link to the applet: http://www.springworldgames.com/eartraining/
I tried to use MidiSystem.getSynthesizer() and it worked great on my system but not on many others'. No sound was played at all.
Then I read about this issue and the fix was to use getSequencer() instead and only add midi events with Sequence and Tracks. Bizarre...
Well, I have implemented that now and it would be very nice to see if it works on your systems
BTW, do you know if there are any issues with playing sampled sound in Applets using SourceDataLines? If there are no such issues, I plan to use Gervill with a bundled soundfont instead of relying on the Sequencer to work.