Audio problem: When Axel first reunites with Serge, Serge greets him with a kiss on each cheek. Problem is, we can hear the smooch on each cheek, but Serge is speaking at the same time, and he does not pause his speech for the smooch.
Audio problem: When Hector Elizondo comes to the park near the end and shoots the guy in the lift, there are a lot of rides operating around him. We don't hear the rides making any noise. Anyone who has been to an amusement park knows how loud rides are.
Answer: Just before Uncle Dave is shot earlier in the film Axel figures out that the note he was given was written on treasury mint sample paper - I believe it's at this point that Axel know Fulbright was in on it since the "matter of life and death" note was written some time before the treasury mint paper truck heist.