While popup notifer pops up out of your system tray, it plays pre-loaded notification sound to get your attention. If you like you can change this sound or disable it. Following shows how to do that:

• Open Preferences Dialog
• Select 'Notification' tab
• Based on your preferences:
  • If you like to disable it, make sure that the checkbox named 'Play following notification sound file while poping up' unchecked.
  • If you like to change the sound file, check that mentioned checkbox and browse and give the location of that file via 'Browse' button.
  • To test whether it plays, you can use 'Test' button
• Click 'Apply' button to apply this change; otherwise click 'Cancel' button to cancel the change

To learn more about the popup notifier, check out related help topic.