Veteran singer-songwriter John Prine and bluegrass journeyman Mac Wiseman hardly act like legends on Standard Songs for Average People, an amiable, relaxed series of duets that’s handily one of the most charming things either has ever recorded. An affectionate, autumnal tribute to 20th-century country, pop and gospel — with its most recent composition, Tom T. Hall’s “Old Dogs, Children and Watermelon Wine,” dating to 1972 — it’s a gentlemanly album whose settings could almost have served during the pre-Nashville Sound era. Prine’s rough-hewn, homey voice fits easily with Wiseman’s only slightly more polished tone; the two could hardly sound friendlier on the likes of Ernest Tubb’s “Blue Eyed Elaine” and Al Dexter’s “Pistol Packin’ Mama.” Not to mention Bob Wills’ “Don’t Be Ashamed of Your Age”: “Don’t mind the gray in your hair/Think of all the fun you had puttin’ it there.”