Title: System Hangs When Using JACK With Java Sound Synth
When you start up JACK while using the Java Sound (Gervill) Synthesizer, your system hangs. The hang can also occur if you try to use the Java Sound (Gervill) Synthesizer while JACK is already running.
Cause Of Problem:
JACK and PulseAudio cannot use the sound card at the same time without manually configuring your system so that JACK comes up before PulseAudio, and then you configure things to use PulseAudio Sink of JACK.
Never use JACK and PulseAudio at the same time. Newer Ubuntu releases (13.10 and above) may have solved the hang, but still, JACK and PulseAudio will not work at the same time without doing complicated manual configuration. The KeyMusician Keyboard will warn you if you try to use the Java Sound (Gervill) Synthesizer while JACK is running, helping you avoid the danger of such hangs.
It helps to think of JACK and PulseAudio as 'mortal enemies'. Be aware that Internet browser audio plug-ins use PulseAudio, as does Audacity.
Fix Available (in version):
The hang may be fixed in Ubuntu Linux releases 13.10 or higher. But I've observed this hang to be fixed, and then come back again.