There's already a load of HP fan games out there (about 60 on this site alone). They don't seem to care as long as you're not making money off them (which I won't - this is just for fun/learning and won't be sold anywhere). And JKR has said that she's cool with fan works IF they are clearly marked that way and not commercialised. Obviously I'll use different music, etc compared to the films, so WB can't complain either.