Hello,  I have the DSM-520 with latest firmware.  I noticed that some of the MP3's won't play on it but will play on every other digital device I have (i.e., ipod, mp3 players, computers, etc...).  Most MP3's play okay but some just display and never start playing.  When I look at the dlink media server it says "Transitioning" and just stays like that until I switch to another song!
I thought it might be the computer and media server so I tried others and still same problem.  I even copied the MP3's to a USB drive and they still don't play but again will play fine on everything else.  
Any ideas would be GREATLY appreciated.  Yes, also eliminated that it's not a network issue!  Thanks.