Sound is crucial to the film “The Zookeeper’s Wife,” a period drama based on the real-life story of Warsaw Zoological Garden owners Antonina and Jan Zabinski (played by Jessica Chastain and Johan Heldenbergh), who rescued hundreds of Jewish citizens by smuggling them through an underground system on their property in WWII Poland, right under the noses of the Nazis.

Yet most of what’s heard by the audience remains off screen: the screams of animals, aircraft bombers flying overhead, piano music drifting through a basement chasm. Crafting authentic and accurate sounds for these cues fell to supervising sound editor Becky Sullivan, re-recording mixers Terry Porter and Anna Behlmer, and the members of the film’s sound department.

The challenge was to authentically portray the changing mix of nature, warfare, domestic life, and life in the Warsaw Ghetto, as well as the perilous scheme going on underground at the zoo over the six-year period covered in the movie.

Sullivan, a WWII buff, turned to her personal library of recordings for the fighter planes, gunfire, and other battle elements. The list of zoo sounds that open the story proved more challenging to nail down.

“We had to show the love for these animals,” Sullivan says. “They had to sound authentic, but not get cartoony. We had to create a personality for them.”

The crew recorded a catalog of sounds for featured animals, including Antonia’s purring lion cubs and a baby camel that frequently followed her around. After spot-testing sound levels, a few short moments of vocals were included.

To create the soundscape for a scene in which Antonia saves a baby elephant from suffocating, the team used recordings of a baby sea lion whose breathing patterns worked best for the injured mammal, and added heavy footsteps and anxious trumpeting to indicate the pachyderm’s parents.

For the scenes that depict the refugees in the Zabinski’s basement, Sullivan and her team were looking for ways to extend a sense of dread. Working with foley artists, they captured the sound of creaks on different types of wood. Adding these to the track, the re-recording mixers worked with reverb and delay to differentiate the presence and distance of individuals on the floor above. Porter also used compressed or expanded piano music to indicate the way it leaked into the basement space.

In addition to creating location-based sounds, the team ensured that what viewers hear corresponds to visual cues, including the reactions of actors.

During the pre-mix and dubbing stage, director Niki Caro and film editor David Coulson collaborated with the sound crew, discussing how to adjust the effects to make sure they captured the emotional landscape Caro envisioned. This included finessing the layout of an explosion — from a plane’s overhead flight to the scattering of debris.

“Niki did a great job of shooting directional cues,” Behlmer says. “If something drew our attention, we watched the action and figured out how to orchestrate it.”