SteamOS still isn't really in wide use, and it's very firmly still in beta. Heck, the latest version still carries the disclaimer, "don't install it on any machine you are not prepared to lose." With Steam Music, though, SteamOS is one step closer to being a multifunctional operating system: it takes your locally-stored music and makes it playable directly in the Steam UI. There's a version for Steam's Big Picture Mode as well, if you'd like to give it a shot on your non-Steam Machine gaming rig.
Valve says that random beta participants (culled from group members here) will get access soon, and a desktop version of the player is on the way as well. There's no ability to purchase music, nor can you pipe in music subscriptions/Google Play accounts -- we've asked if (and when) that's coming -- but it's a pretty major step toward offering multimedia functionality on the upcoming Steam Machines. Like every Valve initiative, Steam Music is an evolving one. The intro page says it all: "Our next steps for the feature will be influenced by your beta feedback."