If it's of any use, and as Bob mentioned above, the terms "soul" and "soul music" starts to frequently appear in Billboard Magazine in 1966/1967, mostly in conjunction with the release of Arthur Conley's "Sweet Soul Music".
There are previous mentions of the term though and it's worth noting that Otis Redding's "Sings Soul" LP was released in September 1965 (and that was a fairly high profile record).
I guess the term "soul" wasn't in mainstream use until later in the decade, but I've found the following clippings/ads from 1965 Billboard issues where the "soul " term was used to describe a specific type of music.
24 JULY 1965:
5 JUNE 1965:
23 OCTOBER 1965: