Twenty years ago the sound of Ladysmith Black Mambazo emerged from the slums of South Africa via Paul Simon's Graceland, an album recorded during the death throes of apartheid but in violation of a boycott that paradoxically kept a great a cappella ensemble from its rightful place in history. They have that place now, and their vertiginous rounds and call-and-responses have become as recognizable and indispensable as Jimi Hendrix's guitar or Miles Davis' trumpet. Their musical concepts are just as groundbreaking, so one can forgive their guest-heavy recent album, Long Walk to Freedom, and listen past the thin harmonies of Melissa Etheridge and Sarah McLachlan for the shivering trill of leader Joseph Shabalala and the collective rhythmic wallop. For four decades, it has been a deathless call of freedom.