Johnny-come-latelies may think of Cash as shaky-voiced, because of Joaquin Phoenix’s raspy impression or his own final releases. But even die-hard fans may be startled to hear Cash’s effortlessly deep, full-bodied baritone in Personal File, newly uncovered, secret sessions from 1973 to ’82, wherein he appears to have invented the solo-acoustic American Recordings ethos two decades early. Were these 49 traditionals, spirituals, and spirited originals meant for an LP? Or was he just scratching an itch? Either way, what a profoundly personalized posthumous gift.