Sirius is pronounced like the word 'serious'. My younger cousins (big HP fans) and I had an on-going debate about this because they pronounce it Cyrus. But if you get books on tape, it *is* pronounced like 'serious' and the movie comes out June 4th so then you'll definitely know! If you get the books on tape,*everything* is pronounced correctly because the narrator sits down with JK Rowling to find out for sure... so this is your best resource for pronunciation of HP words/names.
Also... there is a website that has a pronunciation guide of HP words and names. I'm not sure where exactly it is, but I know it's out there... my cousins told me. :)