The earliest show seems to be the easiest to get seating for. It's not quite dark yet. But we've seen the standby line for the 5pm show start around 3pm, not a good use of 2 hours of park time. If you get there at 4pm you may not have a seat until the next show.
We've stood in the back without seating, stood out near the walkway, even sat at the outside tables by the American Pavillion CS and heard all the music quite fine, just can't see much and can't always hear the narrator unless you are standing right at the back.