I'd like to use the mp3 macro to export 20 WAV files to MP3 (for audiobook upload to Audible ACX)
So i have to use the specific mp3 ACX settings.
So what settings does the mp3 macro line use? (Settings from the last export?)

Thanks!

Yes.

Best to do that as you go. It's also highly recommended to make backup copies in WAV format as you work. It would be horrible if you had to re-record a chapter due to a computer glitch or human error and no backup.