dweide9 The updater app schedules update checks every 4-6 hours (I forget off the top of my head how often). If the updater app is playing music, I'd expect it to play music during the day as well. Also, as you say, it is supposed to be set to play the default notification sound.
I think there must be some other explanation. If this happens again, try checking your notifications or the "active apps" thing (swipe down twice to see all quick settings tiles and there's an icon to see running apps) to see if any other apps are running.