Foley recordings are diegetic sound effects, recorded in a studio and added onto movies afterward in order to increase the realism. Diegetic sounds are those that are supposed to have emanated from the onscreen actions. They might include feet crunching on gravel or the "clunk" of a glass being placed on a table. While Foley was generally recorded as it was needed for each piece, as the studio system became more refined throughout the first half of the 20th century, libraries of Foley sounds were built up, resulting in recognizable specific sounds being re-used in numerous films. One example is the "Wilhelm Scream." Originally recorded for "Distant Drums" in 1951, the scream eventually worked its way onto dozens of movies over the following decades.
Stylistically, many of the darker noir films that became popular in the 1950s used a lot of "stabs," dramatic orchestral crescendos that marked shocking or violent points in the story. Sometimes they were incorporated into the musical score, at other times they were a stand-alone musical phrase that was unconnected to anything else. Many sample packs and audio collections that mimic 1950s sound effects include a selection of "stabs."
Musique Concrete is the art of creating music from "Found Sounds," recorded out in the world. It was popularized in the 1940s and '50s by early sound artists such as Pierre Schaeffer and Karlheinz Stockhausen. The advances in tape recorders made it practical for these recording enthusiasts to piece together noises of traffic, birds, machinery or crowds into experimental compositions. Movies of the period would often incorporate relevant environmental sounds into frenetic montages of activity as soundtrack engineers became influenced by the European experimentalists.
Early analog synthesizers became readily available toward the end of the 1950s and married perfectly with the cultural fascination with all things unearthly. The rash of movies and television programs concerning "Flying Saucers" and "Moon Men" provided the perfect place for the futuristic bleeps and theremin-style sounds that early monosynths produced. Recently, NASA's Cassini probe from Saturn picked up radiation emissions and transposed them into the audible spectrum. The result is strikingly familiar to a 1950s Sci-Fi B-movie, validating the early composer's evocative interpretation of "space sounds."