BioShock Infinite's secret audio was part of audio director Pat Balthrop's experimentations with sound and ambience, Irrational Games told Polygon.
The game contains several auditory oddities, from reversed lines of poetry from Romeo and Juliet to an acoustic play session lowered in speed. You can see the latter of the two demonstrated above by Polygon's own Arthur Gies by skipping ahead to the 1:03 mark.
Speaking with Polygon via email, Balthrop said the game's ambient rumblings were a part of a foray into audio textures.
"When I make ambiences I can get a bit experimental as I seek textures I find interesting," Balthrop said. "Live instruments and melodies were recorded, and then I pitch shifted the song down. I loved the textures that emerged when I did that. I am shocked that someone deconstructed how I made that sound!"