Sue Mingus keeps unearthing obscure treasures from her late husband’s closet, and multiple live recordings by this sextet have been available for decades. So why is Cornell 1964 a must-have Mingus disc? Because, as jazz critic Gary Giddins explains in his spot-on liner notes, “Here is the sound of Mingus elated,” a rare thing for this legendarily irascible genius. The result is a combustible, daring, harmonically dense, and innovative concert with an additional radiance of joy. Eric Dolphy’s bass clarinet is especially perceptive and incendiary on two half-hour-long sociopolitical bonfires, “Fables of Faubus” and “Meditations,” and the sextet lends an ecstatic woof and glide to Billy Strayhorn’s “Take the ‘A’ Train.”