I'm sorry but its a long time since i made the MP3 Player so I am not sure.
However, i have had a quick look at the code - in the listbox when an MP3 is dropped on it - the movie (MP3) player is reset/reloaded.
If you play around with the code in the IF THEN statement part that handles the MP3 DROP - perhaps add some debug messages then you should be able to get the behaviour you require.