Defining aboutness of a collection is a challenge. From a philosophical point of view, this is even harder for collections in anthropological linguistics. These kinds of collections are not assembled for the sake of their "about-ness" but rather for the sake of their "is-ness". A collection in a museum might be about 19th century trains but such collections rarely contain the trains themselves. So, does this mean that linguistic collections are really about the people groups the speech is representing? and then the of-ness is the speech? Then linguists come along and write about the grammar of the language, and that is about the language? Often original stories will have an aboutness meaning which is never recorded in metadata. This needs to change.
This thought needs to be explored with MARC 655 $x and $v sub-fields. see: https://www.loc.gov/marc/bibliographic/bd655.html
see email: https://mail.google.com/mail/u/0/#sent/FMfcgzGsmrDLzSSBqXVPfKphwmdGhcZC