We did this last trip and the audio was perfect. We stood on the left hand side little balcony thing (if you're facing the castle). Our view was obstructed by trees and the flag pole. It was a fine viewing spot, plus we got there about 10 minutes before the fireworks started, but it wasn't perfect.
We've seen RDCT about 10 times, so we're not always looking for the perfect spot, if you've never seen it before, I might recommend maybe staking out a little early near the hub. It's definitely worth it..