more data...
Just realized I ripped 80% of my albums with a another drive and therefore I have a 'backup' or copy. I tried some of those and they play. This other drive is not a fancy raid drive.
Looking at the time stamp on the files, it appears that something happened on 3/22 at 11 am. All of the files that won't play have been 'touched' in this time frame.
To me, this seems something that the drive did to the files. I can send in some files for evaluation if it helps.
In the mean time I do not trust the NAS until this issue is understood.