federico <xaero(a)inwind.it> writes:
  it happens randomly, while I play a soft synth, that
jack XRUNs once,
 and the audio become crappy, noisy, like a kinda of digital
 effect....bit crusher?
 then all synthesizers (all audio apps) sound the same sh*t...
 i tried recording with ardour the output, and the output is clean, so
 I guess is a problem of sync or something that has to do with the
 hardware (soundcard). 
This is likely a hardware problem.
Have you reported it on alsa-devel?  What card do you have?
--
  joq