The way I'd handle this would be to tell the play that I have an idea and I'd like his blessing to run with it. I'll provide something, but your PC will not start off aware of it.
Then, during the session, I'd have a soundboard that I can access to pick sounds at the touch of a button. There are many available. Then I'd fill in many buttons with the basic sitcom sounds - clapping, howling laughter, ooooohhhhs, awwwwwws - and then some from horror movie audiences - screams, etc...
Then, I'd layer onto some of them one voice (a modulated version of my voice) that provides a combination of useful and misleading information delivered by a heckler. For example, you'd hear the sound of an audience laughing, but faintly inside of it would be a voice saying, "Check under the bed, idiot!"
I'd layer these in as the game goes along without explanation, beyond making it clear that the only one hearing anything is the PC. It seems to all be in his mind ... except for the fact that there are useful hints hidden away in some of the sounds.
Lore-wise, the explanation would be something along the lines of a curse, a haunting, etc... The idea reminds me of a 'broken jar' storyline I ran years back where a PC was carrying around an item hiding a 'Magic Jar' gem where the version of the spell used was faulty - and the attempts of the trapped being to take control resulted in sleep walking, hallucinations, and inexplicable knowledge bonuses. I might lean in that direction again and have the being trapped be a bard that has gone insane due to being alone for so long in the item.