Audio problem: In the scene near the beginning of the film where Charles and Scarlet are heading to the first wedding, they miss their turn off and so have to reverse on the motorway. You see that Scarlet turns her head around to see out the back of the car and can hear her swearing but her mouth never opens.
Audio problem: After the first wedding, when Carrie talks to Charles, some weird dubbing comes into play, and their voices become slightly out of sync with their mouths.
Answer: A groom can choose whoever he wants as best man. As you say yourself, it's by no means mandatory for it to be the groom's brother, even if they are close. In this particular case, it's possible that David felt that he ought not to be the best man, due to the difficulty that he'd have in making a speech, but, really, Charles was under no obligation to choose him.
Tailkinker ★