that song is not from some "royalty free music" catalogue and is far from bottom of the barrel, it's a rare single by an artist and is the only record she put out, it's pretty desirable to soul collectors/DJs and has not been reissued...
the only reason they wouldn't have to pay rights is because it is on an obscure label that hasn't been bought out by a major or reissue label (apparently the original masters for many of the labels releases have since been destroyed by a fire), though it is possible the original owners family is still around and aware enough to get paid.
I know what people mean when they say some soul doesn't work well with parts, but I thought this song worked well enough here.