Every time I use the speaker dock, there is a picture of headphones in the notification area on my Xoom. When I click on it, a song appears and I can't close the music app.
There is a small "X" icon when you tap on the music. Just select and close it.
If you have speaker doc and have ever opened your music app there is no stop button. This is a issue with Xoom. You have to force closed the app.