Словарь не дает пометы formal.
This is not my problem! I don't live 'by the dictionary'.
I think I need to change my signiture. When I first replied to the thread I made it clear that I'm not providing any reference to any of Grammar in Use books. I guess I should've added: neither there'll be any reference to dictionaries, research sites, etc
BTW, in the given context I would use 'realise'. I realised I wasn't pronouncing 'X' correctly.