As I write this, there are 45 upvotes for the C Major backing track exercise feature request. Awesome. It should happen.
It would be even better if the app had a whole section dedicated to backing tracks, and you had many more scales to choose from than just the C Major scale.
This feature could use the MIDI sounds.
The main point is that the student has something to play against that they can hear when they strike an out of key note.