Steam is making it easier for their users to seamlessly join their own personalized soundtracks and playlists into their gameplay experience.
Starting today, users will be able to play their own music with Steam Music which gives the option for users to connect their own music collection with their Steam Library. Music playback controls will now be directly integrated into the Steam overlay to allow for marginal interruption of whatever game you may be playing.
Following this news — all of the Half-life and Portal soundtracks are already available for no charge on the Steam store and all the actual games themselves are on sale (75% off).
Official Steam news story