well, after looking at the file properties of the files that would play and comparing them to the files that would not play, I found the problem. I had switched to the newest Amazon music downloader and it was not marking the files under their security tab with access/user name "WMPNetworkSvc", which is actually a service that lets the network see the files via WMP. All my other music files (downloaded, ripped from CDs, etc.) had that service allowed under the security tab.
So--I deleted the 30 or so files I had downloaded with the new downloader, downlaoded the old Amazon MP3 Downloader, went to the Amazon Music site and choose the songs to download again. This time the old Amazon MP3 Downloader picked up on it and downloaded them with the correct WMPNetworkSvc security, and voila, the songs now play through Media Share.
Obviously very few people have had this issue, but if anyone looks and finds this post, maybe it will help someone in the future.