Even though I don't believe the Diner sequence was intended to be comedic, it's hysterical to me, largely because my wife and I have taken part in some friendly bantering about my CD filing system. I file movie music that is mostly or completely by a single artist, such as Mark Knopfler's Local Hero, under the artist's name instead of "Soundtracks" and this drives her crazy because she can't always find the disc she wants to play. However, it makes perfect sense to me.
My second favorite scene is from Nick Hornby's High Fidelity (2000). When the staff of John Cusak's used record store humiliates a customer coming in to look for Stevie Wonder's "I Just Called To Say I Love You" you snicker and nod because you know the type. Jack Black proves just how insufferable music snobs can be. You can watch the whole scene here. (Embedding has been disabled for this clip).
If you can come up with some music related scenes from films that can equal or top those two I'd love it if you would share them with everyone here.
Here's another great scene from High Fidelity with Jack Black leading the way again.