This will please big picture mode users and SteamOS/Steam Machine users alike, Steam is planning to add music controls built-in to Steam itself allow you to control music while gaming.
This is really awesome, considering there are plenty of games I like to play while listening to music and it does become a pain to alt+tab (which doesn't work with a fair few games!) to adjust music.
The full info can be found on this announcement
You need to join the Steam Music
group to get into the beta.