If there's any way you can simulate the PA system at home or even better, the location you'll be showing, that will help greatly! Also ride with other horses in the arena to see how he acts around them. Many horses will speed up considerably with other horses around, either to "catch up" or "beat" the other contestants. Mine did in the only show I've taken him to so far, but WP is NOT his discipline, just wanted to get him used to the atmosphere like you! The thing he was most concerned about was the PA system by far, though, and he's not spooky in the least. Even if you have a small amplifier/radio, you can have a friend speak into a microphone or even play voice recordings, just make sure it's voices and not music so it's more authentic. Hopefully you can make it loud enough that you can hear it clearly across the arena, but anything helps.
That's all I can think of for now, but if anything else comes to mind I'll let you know! Best of luck and most importantly, have fun!