Hello all!
     Now , I have two media store device(HardDrive and USB),so I created two new 
trksession by calling mme_newtrksession() seperately.
     Otherwise, before I use  the one of trksessions, I unset the other and set the current one,such as :
     mme_stop(mme);
     mme_settrksession(mme,0);
     mme_settrksession(mme,current_trkid);
     As a result,when I play the media (one of the devices),it tips there are no data to play,why?
     please give me some advices ,thanks very much!