As part of the Steambox, and Steam Big Picture mode, Valve have announced Steam Music a feature for these two platforms which will allow users to listen to their local library of music whilst in game.
Once you tell Steam where to look for the music it will present you with a tiled interface of either albums or artists, and then allow you to scroll through to choose what to play.
Once at that stage you will be able to play albums and tracks, manage your queue or create and mange playlists; to access your music you just have to press home on the controller, or use your keyboard and mouse if so inclined.
A beta version of the Big Picture version and Steam OS is soon to be launched, and those wanting to be on the list had best join the Steam Music community as beta testers will be selected from the group itself.
You can find out more over at http://steamcommunity.com/groups/steammusic