But detecting “oh, this is voice” is easy. Recording the time where that happens is also easy. Knowing when people are chatting around the phone, and roughly what the fundamental frequency of their voice is, could make $0.001 per person. At Google's scale, that's worth it.

So long as they don't get caught, anyway, because that's all sorts of illegal. Especially at Google's scale. (I don't think Google does this… probably.)