IF the midi is from external midi devices (not vsti's) then you will need to rerout the midi. You can do this by arming an audio track to record, and hooking up audio outs from your midi device, into your inputs with your mixer/soundcard. Hit play and you will have the midi coming out of your midi device as audio and recorded onto the new track.
If your using vsti's for midi then skip that and go directly to:
Export your song into a single .wav file. Make sure that you have "convert midi to audio" box (or something worded similar) checked and it will convert vsti midi into audio at export.
Good luck.