If other people (ie the murderer...) need to hear of it, then it can't just be dumped on him like this -- unless the other people are literally following him, they need to have read about it in the papers or whatever. I don't know what kind of press releases they give for the parties, but they're definitely not arranged at the drop of a hat, even for indy films, so I'd expect PR
noises coming out well in advance so the paps can get there ready for the shots of the guests arriving.
I forget, is he a big name, or only in his own mind? If he is, then he'd have been doing PR
for the film in the weeks leading up to the release -- those are again scheduled well in advance, since they'll want the magazines to be loaded with pictures, and there are issues with lead times for publication.
Why has the agent dropped it on him like this, with only a day's notice? Can't you bring it into the scene with the voice acting -- make the agent's email also a reminder of the party or something? If he's boasting to Jenn about how marvellous he was in the indy film, that helps to give a back-up to his decision not to do the voice work.