It's an old joke.
If ask if you know a certain song, and you say "No, but if you hum a few bars..." meaning if I hum a small section (a few measure, or a few bars) of the tune, it may sound more familiar than just the title alone.
Applying that meaning of "Do you know...?" or "Do you remember...?" to things that have nothing to do with music is the basis of the joke.
"Do you remember if I unplugged the coffee pot?"
"No, but if you hum a few bars..." -- a joke as if there were a song called If I Unplugged the Coffee Pot.