On Tuesday 15 February 2005 1906, somebody named Henry Hollenberg inscribed 
this message:
$ jackd -d alsa
still gives me alot of
**** alsa_pcm: xrun of at least 2.398 msecs
**** alsa_pcm: xrun of at least 9.354 msecs
**** alsa_pcm: xrun of at least 64.750 msecs
**** alsa_pcm: xrun of at least 59.304 msecs
**** alsa_pcm: xrun of at least 4.890 msecs
Try increasing the -p and/or -n flags, and/or using --softmode.  'man 
jackd' will explain these options in more detail. :^)  I use something 
along the lines of
jackd -d alsa -r 44100 -n 3 -p2048 --softmode
which generally gives a minimum of xruns under general usage.  (Athlon 
1800+, only 256MB ram :( )
I've found that a handy way to test for sound output is with xmms's jackd 
output plugin.  I also find qjackctl to be rather useful for 
monitoring/configing/etc. jackd.  It gives a nice interface to start and 
stop the daemon, a message console, patchbay, and will save multiple 
configurations for quick recall. :^)
NRH