From my point of view. You can not play the audio on the xbox in the background. Because, when you are playing games you can hear only gaming sounds. Game sounds are important thing while playing the game. You can enhancing best experience while playing games.
Yesterday, the Xbox One was shown in the ‘stream to’ section. When this is the case, yes you can have background play. Whereas today, for an unknown reason, it’s not available to stream to, thus no background play.
Yes, you can.
Download the VLC app, then you go to the Browse tab inside the app and select “Stream What You Hear (UPnP)”, a Music folder will open and there inside will be two files, select the .mp3 one and you are set.
You can go back to your game and keep listening to the custom sound.