MIDI itself has nothing to do with audio, it's very simple performance data.
What is your sound source for the MIDI tracks?
Are you using Logic's internal instruments or an external synth keyboard/MIDI module?
If using Logic instruments, go to Logic's HELP dialog and type Bounce
If the latter, you will need to record the output of a keyboard or module, Logic can both playback an extrernal synth and record it's output at the same time. The the stereo audio output of the keyboard would have to be routed to the analog inputs of your sound card.
Great Pancenter, Thank you for that outline of options too. What I did was to bounce the tracks to disk, then I opened the files in Adobe Audition where I did a multitrack audio version which was then easy to save to an mp3 file for posting on the web.
PS, sound source is virtual instruments in Logic Pro 9 played using my KORG SV-1 as a midi controller.
AU or VST instruments (Virtual Instruments) are a specialized category as the accept MIDI input yet output audio, in a sense, just like your Korg, the difference being they output their audio directly into Logic's audio engine where they can be bounced just like you were bouncing a group of recorded audio files.
You could've bounced directly back into Logic and done the assembly work there but I understand using an audio editor to work with mastering files, I usually do the same transferring my bounced tracks to a PC running Soundforge.