The proton pack from 1984’s “Ghostbusters” is back in “Ghostbusters: Afterlife.” Used in the original film by the team of paranormal sleuths played by Harold Ramis, Dan Aykroyd, Bill Murray and Ernie Hudson, the pack’s gun blasts a bright stream of light that sounds like a sizzling electric current followed by an explosion.

That’s just one of the few aural easter eggs the film contains.

Director Jason Reitman needed help on the teaser trailer for the new film, which opened Nov. 19, and contacted re-recording mixer Will Files. “The idea was this girl finds a proton pack in this abandoned barn,” says Files. “She doesn’t know what it is, and she starts it up. You hear it off-screen, and it dies. [But] by the very end [of the trailer] it comes to life.” His first instinct was to dig through the sound archives at Sony to see what material existed from the first film. As it turns out, Columbia Pictures did a really good job of archiving, and Files discovered a treasure trove of sounds. There were also digitized work tapes from the original sound designer — as well as outtakes that had never been used in the first film.

Those outtakes sounded like part of the “Ghostbusters” world, so Files used some in the teaser. Phoebe (Mckenna Grace) turning on the pack for the first time was one of the very first sequences Files created. But since the pack was supposed to be 30 years old, he took the original sounds and added new notes that made it feel rusty and miscalibrated. “It hasn’t been turned on in so long,” he explains, “it needs that shake-up and for the oil to be redistributed.”

For the spinning of the pack as it’s starting up, Files recorded an old railroad welding turbine from the 1800s. “We did a lot of experiments taking those original sounds and made new sounds of them. We took the wand and proton stream sound and processed it and made it sound like it was spinning around. It’s what you hear when you’re up close on the back of the pack and you see the light,” says Files.

Another aural Easter egg, Files reveals, takes place when Phoebe slides down a fire pole into the basement. “We took the sound when Dan Aykroyd slides down the pole at the beginning of ‘Ghostbusters,’” he says. “We just stole it from the original and we put it into Phoebe’s [scene].” 

Bringing back the terror dogs, also featured in the original, presented a greater challenge. When he came across the soundtrack, Files says, the quality was low fidelity. So he used the original effects as a foundation. “We found sounds and tried to guess what the sources were,” he explains. “We guessed they were lions and tigers and started playing with them, augmenting them, and we also put our own voices in, pitched down to blend with the animal sounds.”

Files says that one of the first sounds heard when the characters are introduced in “Ghostbusters: Afterlife” is a terror dog crunching on dog food,  but when Reitman heard the sound, it didn’t seem violent enough for him. “What you’re hearing,” Files reveals, “is actually the sound of a lion eating an animal.”