Well I had seen the first film before reading the books so I just took the pronounciation of Hermione from the film.

It took me ages to accept Sirius was "serious" though! *lol* When I was reading the third book, I thought it couldn't possibly be pronounced "serious" so I kept reading it as Si-rus.

I also read accio as A-see-o

But I just assume the film pronounciation must be right