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.