I either just go ahead and pronounce it like it's said in the movies or else just how I decided to say it! I'm always wrong with my pronounciation though! *lol*
To me, "Sirius" couldn't have been 'serious' so I said "Si-rus" (But I now say 'serious' since seeing the film).
I thought 'Accio' was 'A-see-o'!
I thought "Pensieve" was just like, 'pen-sieve', then in the film Dumbledore goes posh with it and says, "Pen-seeve"