I would try renaming the sound files referenced in shutdown-dialog.sh. Then the script won't be able to find them, so no sounds will disturb the library's quiet.
I don't know how long the script will look for the files. Hopefully, the script will give up quickly.<g> It's worth a try and let us know what happens.
If you rename to something like xshutdown.wav, you can quickly change back to shutdown.wav when you actually want to hear the sounds.
Also, if you'd prefer different sounds for startup and shutdown, just rename the files to something like originalstartup.wav and copy your desired sound file to your ~/.icewm/sounds directory and rename it startup.wav or shutdown.wav.