Checking offerings by Abebooks and Alibris turns up odd volumes.
The three music volumes are:
II, Ballads, edited by H. M. Belden and A. P. Hudson.
III, Folk Songs, edited by H. M. Belden and A. P. Hudson
IV. The Music from the Ballads and Songs. Edited by P. Shinhan. This volume has musical scores for most of the songs and ballads.

Mine were listed in Abebooks and were discards from the Fort Worth Public Library (anything not regularly checked out of libraries is discarded). Joe saw the listing and alerted me. Prices were reasonable (probably less than new printings would cost).
abebooks.com and alibris.com

Thanks, Anglo, for the link. There are some other interesting books on that list....