finally, I managed it:

first, I commented out the sound lines in 45xsession and xsession

second, I replace these files via knoppix.sh:

Code:
 # no startup sound
 rm -f /etc/X11/Xsession.d/45xsession
 cp /cdrom/KNOPPIX/45xsession /etc/X11/Xsession.d/

 # no shutdown sound
 rm -f /etc/init.d/xsession
 cp /cdrom/KNOPPIX/xsession /etc/init.d/
works perfect! (don't comment out the 'sleep 2' in the 45xsession)