Well you can't go wrong with creepy organ music in the background. It's a good filler too if you want the audio on a loop. As for the words, if it was me I'd keep it simple. Something along the lines of "Welcome to (the name of your haunt), we hope you return - if you make it out alive!"
Dun dun dun.
Maybe a little maniacal laughter, a few chains rattling, can't go wrong with that!
If you need it to repeat and you don't want one phrase to grow old you can add a few more lines in between pauses. Perhaps some more warnings like:
"Beware all who enter here"
"Stay on the path if you don't want to join the ghosts in the cemetery"
Let us know what you settle on. We'd love to hear it!