1. Apparatus for demodulating a multiplexed sound signal comprised of a first sound signal formed by the sum of a first channel signal and a second channel signal, a second sound signal formed by the difference of said first channel signal and said second channel signal, and a third sound signal formed by the sum of a third channel signal and a fourth channel signal, comprising: sound output means; matrix means for producing one of a first stereo sound signal and a second stereo sound signal in response to said second sound signal and one of said first sound signal and said third sound signal, respectively, and for supplying one of said first stereo sound signal and said second stereo sound signal to said sound output means; and means for supplying said second sound signal to said matrix means; first switch means for selectively supplying one of said first sound signal and said third sound signal to said matrix means.