Trying to get iOS 4 iPod app on a jail broken iOS 5 iPad without totally downgrading
Sorry to disappoint you, but there is no way of just getting the Music app from another version of the iOS. If you are so concerned then please try the App store for Music player apps, i have used iAlbums, it has a very nice and slick UI. It uses the default music player's backend to play music, so you won't lose on any features. 

If you are ready to do some experiment, then you need to build a custom firmware (cooking a rom) with new iOS and old Music app, non-advisable since you risk bricking the device. 

Hope i made some sense.