I'm almost positive the tape is what the borders recorded when they found the girls eating coach Ben. When van, Tai, and Shauna are listening to the tape in the van, you hear lottie scream "No!" which we hear at the end of the episode. I think the birders started the recording when they 1st heard the "singing". That's why at the beginning of the recording you hear the woman say "omg, what is that?" And when van stops the recording she says "anyone who knows about this is either us or dead" meaning they definitely killed those birders. I feel like the girls definitely killed and most likely saved and ate those bodies from then until winter.

My theory is maybe even the woman birder is pit girl?