Given that Haunted Mansion has no music I figured that having some ambient sounds would fill in those gaps nicely. I've started using them when playing and it makes the game feel considerably more immersive. I'll link a few below that I like to use but if you look up "Haunted Ambience" on YouTube you'll find plenty to pick from.

https://www.youtube.com/watch?v=58dAcjgtfbk - sounds more outdoors but is my favorite to use
https://www.youtube.com/watch?v=sCgqiOzXE-g - wind, rain, thunder and fireplace
https://www.youtube.com/watch?v=feorH2u1lME - general white noise