The original soundtrack album — released by MCA Records on April 30, 1997 — features over seventy minutes of the film's music, including some material that was unused in the film's final cut (e.g. "The Hunt"). The single disc is packaged in a paper case that opens to reveal dioramas of jungle scenery and dinosaurs from the film.
For this score, Williams largely avoided using the three major themes he had written for Jurassic Park. Quiet strains of "Theme from Jurassic Park" and the so-called "island fanfare" (Journey to the Island) are occasionally heard, with one scene even including a more robust statement copied note-for-note from the Jurassic Park cue "Jurassic Park Gate." But the original "Theme from Jurassic Park" and "Journey to the Island" aren't heard until the film's final scene and end credits.
However, Williams did write two new primary themes for this score. The first is a rousing melody typically given to the horns, trombones, and strings over a churning accompaniment provided by low woodwinds and percussion. This theme is darker and less overtly heroic than the "island fanfare", but its role is similar in that it mainly underscores the expedition's adventurous nature. Williams intended to use this theme only four times in the film: for the arrival on the island, for the departure from the island, in the film's final moments, and in the end credits. But statements of this theme were tracked into several additional scenes so that it is heard more often than originally intended, and the version heard in the end credits (simply entitled "The Lost World") has been adapted and published for concert performances.
Less memorable but much more frequently heard is the film's second new theme, a haunting four-note melody. The film opens with this motif, and it frequently returns to convey Isla Sorna's dark, mysterious nature and the dinosaurs that inhabit it.
The score to The Lost World takes on a very different tone from that of Jurassic Park. The first film balances tense action scoring and horror elements with a sense of wonderment and awe, but the sequel is mostly devoid of the latter. Williams elected to play up the film's jungle setting with a variety of percussion instruments (including congas, bongos, "jungle drums", gourds, log drums, and tabla). These are heard during many scenes on Isla Sorna, sometimes as a quiet rhythmic background and sometimes as an intense, grooving foundation for brassy action music. The brooding, tropical atmosphere is further enhanced by other instruments, such as shakuhachi and "animal sounds" played by a synthesizer. The themes described above are present in some cues, but much of the music is not based on any identifiable theme.
Another common stylistic element in this score is aleatoric writing. To create a sense of chaos and terror, Williams provides a series of pitches to a group of instruments and instructs them to play them quickly ad lib for a given number of measures. Although this technique has been used in many scores by Williams and other composers, The Lost World employs this effect with unusual frequency, particularly for scenes involving the compsognathus.