According to Ben Kuchera from Penny Arcade Microsoft have confirmed that Xbox One will support DLNA streaming and audio CD playback. Users will not be able to transfer MP3 files to the console, but they will be able to play music from their Windows device (Windows Phone, Windows 7, Windows 8 etc.) through the Xbox One.
All in all, a big yay for using custom soundtracks in Forza Motorsport 5. Apparently no confirmation for using custom soundtracks as of yet.