Video by PG Music Inc. via YouTube
Source
This video goes through the steps of changing a MIDI track to an Audio track, if you already have MIDI notes generated onto the track. Please note that these steps will erase all notes on your selected track. So, if you want to keep the music on this track, it is best to continue to work on a separate, empty Audio track listed bellow your used tracks.
You can find other tutorial videos for RealBand here: https://www.pgmusic.com/videos.realband.htm