User interface sounds stopped working

I have a 2019 iMac running Catalina 10.15.4. For some reason, all the Apple user interface sound effects (errors, trash, etc.) stopped working, though the sound effects for other applications such as Word and Outlook were still active. When I went to System Prefs and clicked on each alert sound, there was just silence. Although rebooting fixed things (just logging out and back in did not), I am curious as to what may have caused this.