Did you actually read the Microsoft requirements for certification?
6.5.1 – Initial launch functionality
When the user is already playing music on the phone when the app is launched, the app must not pause, resume, or stop the active music in the phone MediaQueue by calling the Microsoft.Xna.Framework.Media.MediaPlayer class.
If the app plays its own background music or adjusts background music volume, it must ask the user for consent to stop playing/adjust the background music (e.g. message dialog or settings menu). This prompt must occur each time the app launches, unless there is an opt-in setting provided to the user and the user has used this setting to opt-in.
6.5.2 - Configurable functionality
If an app plays background music, the app must provide the user with configurable settings for both the background music, and the background music volume.
Sounds like your app doesn't do either of these.