I really think it could be done, especially if the protagonist consented to it (in which case he’s actually playing a practical joke on himself). All you need to do is record the voice message using a microphone at the party, and then hypnotically suggest forgetting what just happened. Derren Brown could pull it off, no trouble.
Of course, just because the protagonist has this theory doesn’t mean it’s true. What will he find when he looks in the case?
Ronnie 
Zerrakhi