It's the format of the ingame stadium file e.g. instead of XBOX 360 it was just the old XBOX files as .rx2, hence the change now to .rx
3 file. I think its both the model and texture but i can't verfiy this. I'm sure it's got nothing to do with audio files.
Indeed, the rx2 and rx3 files are even in the same zdata (zdata01.big) in the XBOX360 DVDs and they share the same path directory as in FIFA11 PC (data/sceneassets/stadiums). Same goes for the lighting files.
BTW, I would also love to get my hands on EURO2008 NG XBOX360 stadiums, for NT/club stadiums for Austria/Switzerland, but i've had no luck so far.