More musician jokes (some I've heard other places, some are my own):

How can you spot the child of a bad trombonist at the playground?
He's the only one who doesn't know how to use the slide and can't swing.

Definitions from a middle school band:
downbeat - hitting the musician in the adjacent chair on top of the head with your instrument
upbeat - similar to the downbeat, but achieved by hitting them under the chin
afterbeat - the musician's reaction to being struck, typically followed by another upbeat or downbeat

What's the difference between a piccolo and a chainsaw?
The chainsaw is easier to tune and sounds better in ensembles.

Clarinet definitions:
fff second line G - a musical notation for the entire section to overblow a 24th
tied whole notes - an indication for the 3rd's to take a nap
con sordino - an indication that you have picked up the wrong folder
senza sordino - an indication that you have been playing the wrong part for quite a while
senza boccaglio - without mouthpiece; a technique commonly used among clarinetists with too much free time on their hands