Even if you ordinarily wouldn not stop and linger at Rev. Larry Rice's KNLC-TV (Channel 24), but you can't resist. They're the octogenarian couple singing that beautiful gospel music against a backdrop of fake wood paneling and plastic plants. Slim and Zella Mae Cox have been playing together since 1948, when as young lovers working the cotton fields of their native Arkansas they'd sing up to the telephone lines, hoping their music would travel across the wires and somehow land on the radio. Their dreams came true. These days you can hear the Coxes on the airwaves in five states through the Here's Help Radio Network and watch them several times a week on Rice's station. Slim records the radio shows in a studio in back of the south St. Louis furniture store he and Zella Mae founded back in 1969. "I'm a singing evangelist," says Slim, who taught himself to play music on a used piano at the age of fourteen. "I bought that piano for $7.50, and my first song was 'St. Louis Blues.'"