Max, as usual this is not legal advice, does not establish a lawyer-client relationship, this is just my private opinion, etc.
I disagree with those who advise you to ignore the letter. IF the letter truly comes from the NMPA, I think you should respond (as I gather you are planning to do) saying that you DO have permisson to post copyrighted lyrics, so there! You might add that if any copyrighted words were posted without permission, notwithstanding the efforts of your clearance department, you are very sorry and would they please specify which ones they know of that have been posted without permission ?
The wording of their letter ("as far as we are aware, no authority for such activities has been obtained") suggests that they don't know anything about the Mudcat except that it posts lyrics. It's possible that they have received complaints from some members whose work was inadvertently posted without permission. If they have received specific complaints, of course, it would be a very simple matter for them to specify what lyrics are complained of. But my hunch is that they don't realize that you're already way ahead of them: you've already gotten permission (haven't you?) for most of the copyrighted lyrics on DT. Tell them that, and they may go away.
We should probably cull uncleared copyrighted lyrics from the forum thread archives, at least until we can get a lawyer's advice about them.
and hey! gargoyle is back!