I presume the same strip was in any Sunday comics that carry Non Sequitur by Wiley Miller.

Two old men sitting on a park bench, competing by whistling through their noses.

The sheet music excerpt shown is a real piece.
You should be able to recognize it just by sight reading, but if not, you can actually play it.

I saw that too! Yep a pretty familiar piece.....

Non Sequitur Comic I hope this link works.

Thanks Damon, it does :-)