This is incredible! I have heard this numerous times, especially that gibbon-like "chirping" before the grumbling. One thing that I notices from what I have heard around the US in person and from audio like this is that there seems to be a common "voice" to all of it. I would call it like a giant angry goblin voice even when they are speaking clearly, if that makes sense.
But as far as my answer to the thread goes, I have heard similar talking to the Sierra sounds multiple times, one time it was probably not further than 50 feet from me, there was thick brush separating us but it was still extremely powerful and easy to hear. It was two of them talking back and forth, one a very large male voice and another almost kiddish, clearly much smaller. The voices didn't have as strong of a "oriental twang" to them that I heard though that there was with the ones in the Sierra Sounds. It was very clear non garbled speech and went on for a few minutes. I have heard it in small bursts, including whispers at night, in several places since then but in my experience hearing speech is very rare, I think they only do it when people aren't around that they know of.