When I do a "save copy as" of a .wav file to .mp3...the .mp3 is always longer....Please Please Please, fix this! I'm trying to send both .wav files and .mp3 files to music supervisors/publishers, and they expect a 30 second piece to last exactly 30 seconds, whether it's a .wav file or an .mp3 file...this makes me look very unprofessional!
The problem is that MP3s have always (since Syntrillium days) had a little bit of extra time added, simply because a lot of players managed to cut off the starts and ends of files when they played them. It really doesn't matter on the whole - MP3 is a distribution format, and not a production one, and the last fraction of a second in the environment it is intended for is generally irrelevant.
I don't think you'll get this 'fixed', because there are still a number of players around that will regard this as 'broken' if that happened.
If you need to send compressed files to publishers, explain why you aren't sending them MP3s and use something like Flac (lossless compression) instead. And there are others... Alternatively, use MP2 - broadcasters should be fine with that. There are threads about the best way to do this that you should find if you do a search.