No one can understand people shouting through helmets over their own engine noise. It's a muffled shouty noise that always sounds like anger, even though 99/100 it probably never is. Hard to stop that visceral reaction.
Had some dickhead on a motorbike stop by me in peckham mumbling something, i gave a dirty look and kept walking. only for me to realise it was digger moments later when he pushed his visor up.
I think if you're wearing a crash helmet it's a big NO. You're really hard to understand when you're wearing it, remember ...