Audio problem: When Ritchie is giving his spiel about fandom, when he's facing away from the camera and says "It's about love!", you can see from his mouth movement that he's saying something else entirely.
Audio problem: When Wes says, "What about Deputy Dewey here? Maybe he's the killer," at the Meeks house, it doesn't match his mouth/lip movements in the slightest. (He's facing away from the camera for part of it due to an edit, but you can still tell from his chin movements that he's saying something else entirely.)
Audio problem: When Ritchie tells Sam, "I'm coming with you," right before they leave the bowling alley, Ritchie's line seems strangely dubbed in. It just doesn't quite match his lip movements for the first half.
Answer: As far as I can tell, It's never stated in the movie how he found out about that. Maybe Sam told him off screen when he "learned " that Billy Loomis was Sam's biological father.