A musical pack would go perfectly with Get Famous, and if you're going to have an active job as a musician of some kind, Get To Work could use a little polish too.
I can see why it would take a while. EA would need to hire muscians to create the sound needed - and pay them. All the different levels of ability as your Sim learns, and multiple songs. And if you're playing in a band... well, you have to all be skilled, right? But that's also why I'd like a whole pack dedicated to it. Why do things by halves?
It would be lovely to have more instruments. Drums, but also wind instruments.