HAL did have a baritone voice as he was “dying” - the visual implication of pulling out various modules was causing damage to his ability to talk and then think. The drop in register was designed to add to the unsettling nature of the scene.
Well, yes and no. His voice changed because it was electronically slowed down. In reality, probably by slowing down a tape machine, but that's not the same thing as having a baritone voice, though I might be nit-picking.
