The JACK sound server needs to be running in order to run IDJC.
In order to manually start it try something like:
$ jackd -d alsa -r 44100 -p 2048
If you would like JACK to start automatically when you start IDJC try this:
echo "/usr/bin/jackd -d alsa -r 44100" >~/.jackdrc
If you have already done this it is possible another application or non-JACK sound server is using the sound card.
Possible remedies would be to close the other audio app or configure the sound server to go into suspend mode after a brief amount of idle time.
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neyder@Candelaria:~$ jackd -d alsa -r 44100 -p 2048
jackd 0.102.20
Copyright 2001-2005 Paul Davis and others.
jackd comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
JACK compiled with System V SHM support.
loading driver ..
apparent rate = 44100
creating alsa driver ... hw:0|hw:0|2048|2|44100|0|0|nomon|swmeter|-|32bit
control device hw:0
the playback device "hw:0" is already in use. Please stop the application using it and run JACK again
cannot load driver module alsa
no message buffer overruns