Thanks for pointing that out, Mick.  I hadn't read that article yet, but on referring to it, it makes clear that Karpeles' papers -with the exception of the Newfoundland material, which went to Memorial University of Newfoundland- were bequeathed to EFDSS for the Vaughan Williams Memorial Library (which now also has microfiche copies of the Newfoundland papers).  The documents are catalogued according to the boxes into which Miss Karpeles sorted them before her death, and Box 6 contains her diaries from the 1950 Appalachian trip, and lists of singers and songs; there is also some material from 1955.  It isn't clear from David Atkinson's summary whether song transcriptions from that expedition are included in this or in other boxes.

It appears that some of the sound recordings were released on disc by the BBC; the  VWML  has copies, at any rate, as does  The National Sound Archive,  where you can get details online.  It would seem reasonable to suppose that the Library of Congress might have the original tapes, or copies of them, but at this point Peter Kennedy, Maud Karpeles' nephew and a prolific collector in his own right, enters the equation.  It certainly looks as if he has the tapes, or some of them (and knowing a little about him, I shouldn't be surprised if they were the originals).

He runs  Folktrax,  as you may know, and has two anthologies of material from the 1950 trip available for sale (cassette or CD):

FTX-907 - BLACK IS THE COLOUR: Appalachian Collection -1 (Tennessee and N.Carolina)

FTX-908 - CUMBERLAND GAP: Maud's Appalachian Collection -2 (Virginia ballads and dance tunes)