Yeah, it definitely seems like a WMP problem. However, it does only happen with the DNS-321 shares, like I said. I can play files on shares from another windows PC. I also just updated to WMP11 and it has the same issue. I'm on Windows XP SP3, and just recently ran windows update.
One other discovery I made is that if I go to the full UNC path of where the media file is located, and double-click it, then it plays fine in WMP. So it's like it can't find the file when it's on a mapped drive.
I also just reformatted my DNS-321 and am now using two drives in Raid 0, formatted with EXT2. Just wanted to see if that made a difference, and it didn't.