Field Corp. will purchase the piped music unit from the Westinghouse Electric subsidiary for undisclosed terms. The sale is part of Group W's previously announced plan to concentrate on commercial broadcasting and programming. Westinghouse acquired Muzak in its purchase of Teleprompter in 1981 and said in January that it wanted to sell Muzak. Muzak serves more than 135,000 subscribers with recorded office music through about 200 franchises in the United States and 15 other countries. The sale is expected to take six weeks to complete.