You need to go to . . .
Edit - Preferences - Privacy & Security - Popup Window Controls . . .
Click the radio button next to "Suppress Popups", and then the option to
select a sound for popup suppression, should no longer be greyed out.
From there you can click the Select button, and scour your
computer's hard drive for a sound you think is most suitable.
(Yes, this does work in my DJGM-i Distros of Netscape 7.01.)
As I have Windows XP running at the moment as my default OS, I've
selected to use the "pop" sound that usually accompanies those oh so
annoying nag balloons that appear above the XP system tray, to say
that disk space is running low, or to sign on to Windows Messenger.
(FYI: The sound file I use, is called "Windows XP Balloon.wav")
(BTW: I disabled Messenger in XP as soon as I found how to do so!)
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.0.2) Gecko/20021120 Netscape/7.01 (CK-SillyDog)
SeaMonkey = Swiss Army Knife: It's versatile, reliable, and contains useful tools.
Windows Internet Explorer = Old Swiss Cheese: Full of holes, and it stinks!