Simple, as stated by Chris
1. Recording Button > MIDI Quick Recording > Record and save as Midi
2. Player Button> Select recorded Midi file (or any other midi file)
3. Recording button > Select Audio Recording / Quick Recording.
4. Audio Recording Screen > Select Record > Midi button > Select Play
5. Sing or play along while Audio recoding is recording
4. The mix will be recorded and automatically saved as an MP3 or Wave
Summary: Play a midi file while simultaneously recording an audio file