John,
Just so you know how I tested - I watched a short portion of an AVI (XviD) that I know worked fine on MC and firmware 1.01. I was able to watch at least a part of the AVI in the UPnP servers listed above. I didn't test MP3s in the other servers, and I didn't watch the whole AVI.
My point is that Motorhead may be right that there is a larger problem with the firmware. But my brief testing showed that AVI files would at least play on the other software and not on MC.
Motorhead's also right that the DLink Media Server is less than worthless. Windows Media Connect and Vista Media Sharing are not in any way configurable. None of the three even do a basic Artist-->Album view properly. Without MC, the DSM-520 isn't worth much.
*****************
Motorhead - if you get DLink to make the 1.01 firmware available to you, PLEASE post in the thread. I found a process to downgrade the DSM-320 (using dummy DNS entries and FTP site), but we would need the firmware to do the same to the 520. DLink makes the old DSM-320 firmware available, but I can't find any old DSM-520 firmware.
Also, DLink doesn't actually manufacture the 520 (or they don't write the firmware for it). I'm looking for the name of the company that does and will post here. If DLink won't give up the old firmware, we may be able to get it from the other company.
Edit - the company that makes the DSM-520 is named RedSonic, according to this site:
http://www.suphammer.net/Hardware/DSM520RedSonic:
http://www.redsonic.com/