I had that happen to me the first week I had my car which was about 5 months ago. I called Tesla and we did a reset. It has never happened again. In my mind, it was remotely related to Bluetooth. I think I had gotten a call right when I got in the car and then when I hung up I had the sound.
it's a song that's hung up ... choose any other station on Slacker or a song on USB. it will stop. This happens sometimes when you are in a bad cell reception area and change the song to many times real fast and sometimes on startup when it tries to connect and cannot. I've also seen it when I was on the edge of Wifi and it flipped from Wifi to LTE to Wifi to LTE real quick.