Liz, if you right click on the tray icon for PopupKiller, then click on 'Preferences,' you'll get a three-tabbed box for setting certain parameters. If you click on the 'Look and Feel' tab, there's a box you can uncheck to disable the sounds.

And, kat, if you do the same thing and click on the 'Detection' tab, you'll see a box in which to set the number of windows that can be open at one time. I set mine to 10 and have had no problems.