That took me way to long, too. I'm not from an English speaking country and don't really know the song, so I didn't pay attention to the notes and just tried to find the pun in the text, didn't find one and gave up. I only noticed it, when this post scrolled by me for the second time.
I should have never given up, as that only let me down...
I had to actually busy out my recorder because while I can read sheet music, I can't, like, imagine the sounds they make? I'm pretty sure I used to, when I knew them as DO-RE-MI, etc. I haven't thought of it like that since first learning an instrument instead of just singing in choir, though.