Yesterday (10/10/14), Valve released Music Kits, as outlined in the Counter-Strike Global Offensive blog. According to the blog, they can be received by "offers in-game" or purchased from the market.
Want a music kit? They’re available as offers in game, or you can pick one up now through the Steam market.
What exactly do they mean by "offers in-game", and how do you actually get one (apart from buying them from the market?
I couldn't find a way to get them from cases or any usual key-and-crate method. Neither can I find them being sold in the "store".
NB: The store is that bar above the blog with the cases and such. As per current, it only sells keys and contain info about the crates. Apart from that, you can't buy the Music Kits directly as a microtransation.
Best Answer
The Music kits can't be dropped, but only offers(looks like crates and works in the same way.)
You can buy Music kits on the steam market from other users, or you just have to get lucky and drop the offer in game to got the chance to buy a 6.99$ usd coupon; giving you the music kit.
Once you have drop the offer, and use it, you'll see the coupon, and will be able to buy the music