Now that the video companies are running out of old movies they can put on video, at least two of them have come up with something else to recycle for the hungry new medium: pop songs. Prism Entertainment, one of the larger video companies not affiliated with a major movie studio, has just completed filming the first of a series of feature-length, made-for-video movies based on hit songs of the '60s and '70s. "Hot Child in the City," a No. 1 hit for Nick Gilder in 1978, serves as the basis for a murder mystery involving a "sweet young girl from Kansas" who gets mixed up with the wrong crowd in L.A. Also on the way are movies based on the Animals' "The House of the Rising Sun" (about a brothel) and the Moody Blues' "Nights in White Satin" (a look inside the lingerie business).

While the producers may have skimped on story development -- and we can only wonder what horrors they would devise for "I've Got You Under My Skin," or "I Fall to Pieces" -- they are putting their money where their ears are, lining up songs for the sound tracks from Lou Reed, Billy Idol, Go West and others for "Hot Child," with Tina Turner, Roxy Music, the Thompson Twins and Pat Benatar among the artists whose tunes fill the other two. The producers are aiming for the "distinctive, pulsating, stylized approach" of films that incorporate music video-style clips into the story line as "Flashdance" and "Footloose" did. We'll have to wait until October, when "Hot Child" shows up in rental stores, to see if they succeeded.

Polaris Communications, publisher of Esquire magazine's two video series, has come up with a video to help viewers take a more active role when they relive their musical memories. The "Esquire Dance Away" exercise series invites women viewers to "get fit with the hits" with low-impact aerobic routines built around popular songs from the last four decades. Instructor Molly Fox, a star in some stretch-and-burn circles, has incorporated steps from period dances into the workouts so those inclined can resurrect their Watusi along with their miniskirts this fall. Four 30-minute, $16.95 tapes -- the '50s, '60s, '70s and '80s -- are due in early October.

Yabba-Dabba-Doo

A little gem of television prehistory comes to video next month with the release of the first four episodes of "The Flintstones" on one two-hour tape for $29.95. The first prime-time animated series, "The Flintstones" painted a picture of life for the "modern stone age family," with satirical barbs far sharper than much of the live action that fills time between commercials today. The first quartet is vintage Bedrock -- no Ann-Margrock or Cary Granite here, just classic comic battles in suburban wars between neighbors and sexes. Fred and Barney need an act for the lodge talent show; Fred and Barney fight over rights to the swimming pool they've built; Fred and Barney's bowling tournament falls on the night they've promised to take Wilma and Betty to the opera; Fred gets Barney a job and gets fired himself. Purists will note that the boys' fraternal affiliation in the premiere episode was with the Order of the Dinosaur, not the Water Buffaloes that dominated so many later episodes, and everyone will recognize the voice of the original Betty, character actress Bea Benaderet (of "Petticoat Junction" fame), who died during the show's run.

Three Men

Next month's rental releases feature a triumvirate of Hollywood's most popular actors in three of their least popular films. First up is "Critical Condition," wherein Richard Pryor seeks laughs as an escaped convict posing as an emergency room physician during a blackout. Next, Sylvester Stallone weighs in with "Over the Top," which tried, unsuccessfully, to do for arm wrestling what "Rambo" did for bounty hunting. Finally, Robert De Niro takes to the jungle in Warner Bros.' "The Mission," the last movie that David Puttnam produced before he took over at Columbia. Since De Niro redeemed himself with this summer's "The Untouchables," Warner is cutting the price on two of his better efforts: Martin Scorsese's "Mean Streets," soon to be priced at $24.95, and Sergio Leone's "Once Upon a Time in America," available on two cassettes in all its uncut, 3 1/2-hour glory for $29.95.

Wee Watch

The "Wee Sing" series of children's books and sing-along audio cassettes has long been a favorite of car-pooling mothers and others who have found that a little guidance from the tape deck can make the little ones' voices more tolerable. The first "Wee Sing" video, issued in 1985, proved that it works with a TV set, too. Now the "Wee Sing" crew has another video on the way, "Wee Sing Together in the Kingdom of Old King Cole," based on the "Wee Sing Nursery Rhymes and Lullabies" book-and-cassette package. Jack, Jill, Little Boy Blue and Mary (the one with the lamb) lead two dozen kids through 20 nursery favorites in the space of one hour. Price/Stern/Sloan hopes to get the tapes ($21.95, or $24.95 when packaged with an audio cassette of the sound track) into stores by the beginning of next month.