What we know of the “Muscle Shoals sound” is this: It comes from Alabama, from the banks of the Tennessee River, which Native Americans there know as “the river that sings.” It may have to do with how the lower register of a rhythm section is folded into a pop-music mix. In any case, it is that special something just under the surface in so much vintage R&B, ranging from Aretha Franklin’s “I’ve Never Loved a Man (The Way I Loved You)” to Percy Sledge’s “When a Man Loves a Woman.” Indeed, it has yielded a lot of music for men and women alike to do a lot of loving to. As director Greg “Freddy” Camalier’s discursive documentary Muscle Shoals reveals, FAME Studios founder and stoic family-tragedy survivor Rick Hall is the man most responsible for the Muscle Shoals sound, even if Hall’s house band eventually left to start its own nearby studio by the name of Muscle Shoals Sound. That band was the Swampers, notably “a bunch of white guys that looked like they worked at the supermarket around the corner,” as Bono puts it, emphasizing the remarkableness of just how much essential American soul music those players backed up. Bono is but one of several non-American musicians drawn to the Muscle Shoals sound. It is also something about which Keith Richards often mumbles fondly, wishing his records had more of it. Cut from the same music-doc cloth as Dave Grohl’s recent Sound City, Camalier’s tuneful if somewhat overlong anecdotal history does the most justice to the Muscle Shoals sound just by listening to it.