That sound means you "cleared" the dungeon. Usually this happens after killing the boss at the end. Some places don't have bosses so it just happens automatically. And by dungeons I also mean things like landmarks, caves, etc.
Yeah I guess it must have meant that I cleared something. Of course it's not the tribal dragon fighting music or the music they play when you're near a word of power. I know both of those sounds. I'm level 44 after all. I had happened upon an abandoned tower that had been slaughtered by an attack. I had found a note that said that they were going to be attacked and reinforcements were going to arrive, but apparently they never did because all the imperials were dead. I had decided to camp there for the night, since I was travelling to Riften and it was nighttime. When I had woken up the following morning as I was mounting Shadowmere, I heard that sound. I thought it was bizarre to hear it then.
Oh well.