Jack Audio Not Working

[Chris Spencer]

I'm unable to hear any audio using the Jack server. I've followed the
guide at http://www.passback.org.uk/music/fedora-music-intro/ and
everything seems to work, except that I don't hear anything. If I
connect VKeybd to ZynAddSubFX using QJackCtl, then hitting keys in
VKeybd causes the decibels to spike in ZynAddSubFX, so I think Jack is
working but it's output isn't getting to ALSA.

Another interesting point is that while Jack's running, I can't play
music in any other apps (Audacious, VLC, Timidity, etc). However, as
soon as I stop Jack, sound returns.

Is this a bug somewhere, or could I have something mis-configured? Any
help is appreciated.

Thanks,
Chris