|do you open any other media players up.. at all... or Do you also have that folder on your/a network..|
if any of those are true..
one of the players that you or another user uses & has set folders to monitor.. is set to convert files.. much like renaming and rearranging files if set to.
It can be a real pain.. I had family come in from VA.. connected to my network... next thing I knew some 6k tunes had already been converted before I caught it..
It can be simple matter of just altering the setting to convert them back or find a converter to do a batch job