sounds like (no pun intended) that when you turn off your car, the car sends the play/pause button command to the phone, then turns off the bluetooth connection. This incorrectly starts the music playing through BT, but by the time the music starts, the car audio is off. The phone will then continue playing and after a few seconds the BT A2DP connection is auto magically sent to the only audio output available, the speaker. You can find more on at: specifically AVRCP and A2DP bluetooth profiles.
http://en.wikipedia.org/wiki/A2DP#Au...le_.28AVRCP.29
The only thing I can think of is to keep a emply play list in the sprint music player