41. I see a horizontal list of notes with "Instrument number: 0" on top. This could be played on a piano.

43. Yes you can.

45. internal: "public void playTune(SimpleTune tune) {...}"
    external: "Track track = sequence.createTrack();"

51 and 52: why is are there NumberDisplay questions in part 3?

58. What I'm seeing is the trajectory or the order in which the code executes each line. It goes through many phases, from getting created to executed.