Comment 3 for bug 232341

The problem you were seeing could just be a problem with ZynAddSubFX which doesn't really work that well with jack. It's jack connection pretty often get's "zombified" and it just doesn't produce any sound anymore. Of course in that case triggering notes using Zyn's own virtual keyboard also fails (no sound). Also using Zyn' with jack's "Frames/Period" setting below 1024 usually results in broken sound.

For those reasons I pretty much gave up on ZynAddSubFX myself. A nice replacement for it is WhySynth (http://www.smbolton.com/whysynth.html).