There is
  no way to upload audio into the YouTube Video Editor
 , so only use the audio that YouTube supplies please, and in the way that the course shows.


 YouTube is understandably concerned that creators would upload music, almost all of which is copyrighted, so I cannot see this changing.


 Remember, though,
  you get many tracks and sound effects, all of which you can monetise,
 here



  https://www.youtube.com/audiolibrary/music