The mumbling wasn't intentional, when the production went on tour in the US, many of the venues did not have proper acoustics and as such, the dialogue could not be heard. Many critics considered this intentional, and Brook and the actors guilty of this played along as they were getting poor reviews from American critics as they weren't shocked by the Brook production in the same way that the British were.
You COULD mention mumbling as a technique, but it doesn't highlight any themes or moments in the play, and as such, is an average argument.
