Was duke ellington's 1941 number 1 hit song, which he wrote with billy strayhorn. it was the song you listened to in the sound files for the lesson.

I didn't listen to the files in your lesson, so let's list the information we have:

-song composed or performed in 1941
-song written (together with) Billy Strayhorn
-song by Duke Ellington

There is one such song, and it's "take the "A" train"  - I think this is the correct answer!