I was just goofing around in my redstone-test-world when I discovered this:

As you probably know, A block gives a output via a comparator when its being updated and the block changes, so what I did is put a redstone clock nextm to a jukebox, and get a comparator as output, when I put the yellow music disc in, there was a very weak signal, but as I was trying other discs I siscovered that the output changes in length.
The order in wich the signal changes is the same as it apears in your creative-inventory, just search for music discs. the first to apear (the yellow one) has the weakest signal, the last is the strongest.

Due to the 1.7.2 update it will apear that after a few trys the music won't play anymore, but still its a very cool discovery I believe and its usefull for things as locks.