Exporting to audio?

If the tracks are midi then you need to convert them to Audio to be able to export them as audio.

Also i might ad that it would be smart to export them as wave audio rather than MP3 which is a compressed audio and has some lost value due to the compression process. then one can also make an MP3 at the same time for storage on the internet ot file sending, but Wave file audio is far better to keep and burn as it has better sound.