Interesting that they think there's a single universal voice amongst hens. I've got a small flock of a dozen birds, and they each have distinct voices and calls. You can always tell who just finished laying by which egg song you're hearing, until the others join in and it's a cacophony.
We've got one anericauna who likes to "scream" for her egg song. Awful sound, very close to a hen screaming because a bobcat got her.
I don't think that they think that. All humans have different voices, yet it's pretty easy to tell what mood someone is in, even if you don't know the languages. Why would you assume that chickens express these so differently that it can't be interpreted automatically, when the same is possible for us with much higher ranges of possible expressions?
It wouldn't have to be a universal voice, just something consistent in the noise. Neat thing with lots of AI training is we don't have to necessarily even have a theory of what the AI is picking out. It just has to be in the training data and our hints during training have to be appropriate.