I bought ES2 a million years ago on day one on Steam - Digital Deluxe edition with founders bonus (whatever that means)

That came with the soundtrack, and I've got the soundtrack in my steam files, however...

Currently steam doesn't show me as owning the Soundtrack "DLC".

I guess I'm wondering

1) how I originally became entitled to the soundtrack - assuming it was a founder's bonus but I can't find details to be sure, and more importantly...

2) how I can have that re-credited to my steam account

Soundtrack isn't available as separate DLC, only as part of deluxe, that's why you can't see it in Steam. There is though a separate payable lossless OST, that came out this year, so you might be confusing it with that.

I'm confusing it with something. My soundtrack is 54 tracks, which includes those from the two additional OST releases ...plus some other stuff I guess.

I manually mirror my soundtracks to a cloud drive; my earlier mirror was only up to 34 tracks (the original OST).

So you must be right - some kind of original deluxe edition entitlement.

Consequent DLCs you bought expanded tracklist you had.