Early sound films really had only two elements: sound and music. The sound came from live recordings on the set. If someone dropped a dish in the live-action scene, the sound of that dish was permanently mixed in with ("married to") the dialogue. Afterward, in a separate studio, an orchestra would record the music for the film, and eventually the music would be married to the sound and dialogue in a simple mixing studio. Originally these were mono mixes of elements on so-called dubbers, devices similar to the film projection devices used in theaters, except that they held film stock with recorded sound elements only.
Eventually, as movies and movie-goers became more sophisticated, demand grew for better film sound. Eventually SFX and dialogue were almost entirely separated to allow for more clarity and more convincing realism. While some live sound effects were retained in live-action dialogue, other sounds in films came from sound libraries--prerecorded sounds catalogued for use--and from foley tracks, sound effects performed by foley specialists in a recording studio in synchronization with a film of the live-action.
Beginning in the 1980s, film sound became increasingly digital, meaning most analog sounds originate in microphones, either on a live set or a foley sound stage. The huge 35 millimeter sound dubbers of the past are almost entirely extinct, replaced by racks of computers and manipulated on digital mixing consoles. Sound elements for a typical big-budget film's sound mix have been stored on many disk drives attached to multiple computers and mixed on a large digital console (150 separate tracks or more) on a large sound mixing stage. Two or sometimes three separate mixers mix each of the three main elements: music, SFX and dialogue.
Contemporary film sound is no longer in stereo. There are two elements similar to but not quite the same as stereo, music left and music right. There is also a center music fill track, and a low-frequency track with sound elements lower than a specified pitch, often around 50 to 100 cycles. Additionally, there are split-surround tracks for parts of the music mix that will come from speakers to the left and right of the audience. These elements comes from so-called stems, or individual stereo pairs. A typical mixing session may have these individual stereo stems: brass, winds, strings, percussion, vox (singing and other human sounds), synthesizer, low frequency and other. Certain sounds are isolated onto low-frequency tracks before they are mixed; others are electronically isolated from other stems in the mixing process. There are additional music tracks, among them what were originally live-music sounds that have now been isolated from the original live-recording and processed and pop music tracks to be incorporated into the film mix.
Sound effect elements have proliferated enormously since the advent of digital sound. Pioneers like Walter Murch, who long worked with Francis Coppola, and Ben Burtt who worked with George Lucas, created a new field of sound design, which emphasized the creation of sound effects as part of an overall sonic canvas that took sound effects away from literal reproduction into the realistic sound of Hollywood films today. Individual sound editors may work for months or even years preparing source tracks that will be mixed and remixed long before the final film sound mix. For example, the film "Apocalypse Now" had well over 100,000 separate sound elements. To some extent the creativity of sound designers has blurred the distinction between the three traditional elements: live sound, foley sound and library elements. By the time it reaches the mix stage, a given sound element may have dozens of separate premixed sources, many drawn from all three areas, modified and then modified again.
What has come to be recognized as a landmark event in film sound was an apparently undramatic decision by the Coen Brothers, who decided to mix the film sound for "Oh, Brother" largely within a low-cost computer environment using what had been thought to be a semi-professional sound program, not quite fit for major films. Since then, the industry has moved toward smaller, low-cost music and sound creation environments where the elements are no longer mixed on a sound-stage, but within a computer. While large post-production mixing facilities remain, many sound editors set up their own home studios and to do professional work at a fraction of the cost. Similarly, more and more music soundtracks for television and increasingly for feature film no longer originate on a soundstage with an orchestra but are created digitally in composers' home studios. While many of these home studios cost over a hundred thousand dollars or more, many do not.