The last part "played backwards", to me it sounds like
hey humans, or
hi humans.
I'll
Throw my two cents into this audio thing. Way back when I was 100% sure I heard a voice in the audio. Repeatedly I pled my case but nothing. Soon I begin hearing what I thought I heard the first time change and as time went by I realized that I was wrong.
I was so sure though, and I let that block any other way of thinking and I stalled on whatever we were discussing at the time. Like rizal said a one point, voices are used a lot to alter and make something new. Chances are you just happened to isolate that voice and manipulate back to normal or close to it. While I myself do hear hello, I can also see, having again gone through the same thing, how my brain is just trying to make sense of something strange but close to human.
Sorry if that doesn't really make sense. I guess all I'm saying is maybe we shouldn't focus on them hiding a phrase in the audio. Our brain is great and trying to make something out of nothing and white noise and static is a great starting point for that process to work magically.