There's a sound clip here on WD, posted when the question was asked before.
In it, (maybe it was a Czech-speaker) it sounded more like vul-chak (vul, rhymes with with the beginning part of 'wolf'.
I wonder which is right? I'll try to dig up the link.
ETA: Here:
http://www.wolfdog.org/forum/showthread.php?t=14853