Across the runtime, Evy and Justin systematically process the ten mysterious audio files. In the recordings, Jessa's nighttime sleep-talking gradually mutates into guttural glossolalia, threatening statements in ancient languages, and violent domestic outbursts that culminate in Mike's apparent death and Jessa's disappearance.
As Evy isolates specific frequencies and reverses audio tracks, the boundaries between the audio files and her physical home disintegrate. Whispers, floorboard creaks, and reversed breaths begin echoing not through her headphones, but from the upstairs hallway. Evy discovers that the anonymous sender's email contained acrostics spelling "ATONEMENT" and "TENET," directly targeting her repressed guilt over abandoning her mother during her earlier struggles with alcoholism.
In the climax, Evy sits in the pitch-dark living room during the 3:00 AM session as her mother's room goes completely silent. Upon heading upstairs, she finds her comatose mother’s bed empty. In a single, continuous take in near-darkness, the sounds from the tenth audio recording sync seamlessly with physical movements inside the house. Evy is cornered by an unseen presence that mirrors her mother’s physical form and Jessa’s demonic voice. The ending leaves Evy trapped in the audio loop—either fully possessed by the auditory curse or completely broken by grief, cementing that the true "undertone" was the unresolved guilt she could never silence.