This is really important. My first edit in FCP I put the song in first, and i think somehow it matched the sequence settings to that, idk why it would do that. Now i always just drag a clip over and drop it in, and when the box pops up and asks to change sequence settings to match the clip settings, hit yes. You probably know that already, and it's probably not even the issue. The point is, export settings are a lot more trouble to mess around with, and it usually won't get you anywhere (from my experience). if the sequence settings are all ok, then just exporting with 'current settings' will be fine.