Press the circle button thing on the bottom twice really quick, slide your finger left(on the bottom, where the stuff popped up), and press pause. If that doesn't work, or anyways, click music and check through it all to see if it is playing and try to pause/stop it.
Actually you can't switch off the music on an iphone when connecting to an automobile audio system or similar. Also, to pause it is also a problem because it frequently becomes "unpaused" I have a solution which works well, because I need to have my Aux connected when I use my iPhone as a GPS. I created a playlist which I named "Silence" and downloaded a song, which is to do with Remembrance day in the UK. It is called "2 Minutes Silence" and costs NZ$2.39. I then put that song in the "Silence" Playlist and put it to loop, so in effect I have 2 minutes silence, continuously looped which gives me what I want. Mickey Mouse to be sure , but it works.