You can launch Groove outside the game as background music. Load up your OneDrive account (everyone with hotmail automatically has this) with locally saved music (mp3s). Then launch Groove and it should add your music. Turn down the master volume and enjoy your music. (create playlists if you wish)
The problem for me is there is no mute at all in Forza 7, even when I turn down the master volume, the engine noise is still loud. They should implement a separate slider for engine noise.