MOVIES ON AUDIO CASSETTESare being released by Omni magazine and Utah-based Media Communications. Running 60 minutes each, the cassettes claim to have such vivid sound effects that with their original musical scores, the tapes convey the impact of a film. Making up for the absence of a screen is a secret weapon - listener imagination, which the producers say comes into full play during the listening. First in the series: two stories by the science fiction writer Ray Bradbury: ``And the Moon Be Still As Bright'' and ``Off Season.'' AMERICAN THEME PARK SHOWS will entertain in China for the first time, Nov. 15-29, at the Dong Fang Amusement Park in Guangzhou. Productions of ``Hoedown America'' and ``Allan Albert's Rock 'N' Roll '87'' will give 5,000 people daily their first taste of popular American culture. Producers are Allan Albert Inc. and Hersheypark. THE PHOTOGRAPHS OF ELIOT PORTER will be shown at the Amon Carter Museum in Forth Worth, Texas, on Oct. 31, in the first retrospective of his 50-year career. The photos were bequeathed by Porter to the museum. After Jan. 3, the show travels to Bowdoin College Museum of Art; Brunswick, Maine; and the Huntsville Museum of Art (Ala.).