Recently Steam gave the players the opportunity of buying the digital versions of the video game soundtracks without buying them as DLC.
However it will take some time before the developers convert the existing soundtracks.
That means you do not have to own a certain game to listen to its soundtrack through Steam anymore. That works for the new releases for now.
That will improve managing and downloading music into the new Steam library. However it will take some time before the developers convert the existing soundtracks. However, Valve created a tool that will automate the process but it takes time.