There are a few ways to do it. "Bounce to tracks" being the most frequent used.
I go for a simpler approach and freeze the track.
Simply freeze the Midi track using the freeze icon.
Once it is frozen, drag the content (the audio) to a blank audio track.
Then you can unfreeze the original and still have it for future needs.
Once you freeze a track it is just like an audio track. A few things to be aware of is levels and Freeze options (if you right click on the freeze button).
Or simply select the track and go to Track options in the TV and select Bounce to tracks.
In the "bounce to tracks" dialog, select Tracks.