By the way, a question I've always had about TESB musical soundtrack, is the bell sound right around the time Vader knocks down Luke and tells him he is beaten part of the musical soundtrack or is it just a sound effect? Either way, I've always liked that, too. It reverberates with a tone, so it does sound like a bell.
That bell sound is a sound effect of the two mens lightsabers hitting the railing. I always with Kirshner had given us a closer shot of that moment because it's the only time in the duel when Luke and Vader are actually physically touching as they push on each other.