Yup, I've heard Huun-Huur-Tu perform in concert, and it is amazing. If there's anybody reading this who doesn't know what we're talking about, I've found some sound samples at CDNOW that illustrate this:

Go to Huun-Huur-Tu : Where Young Grass Grows and play "9. Barlyk River". The sound that vaguely resembles a whistle that enters about 23 seconds into the sample is actually a human voice! Another deeper sound appears in "10. Tarlaashkyn", appearing about 15 seconds in.

On Huun-Huur-Tu : If I'd Been Born An Eagle, play "8. Chylandyk". It combines the two sounds that appear separately in the first 2 cuts. But still, that's just one person's voice you're hearing!

On Huun-Huur-Tu : Orphan's Lament, play "1. Prayer" to hear several voices chanting together. I think this is similar to what some Tibetan Buddhist monks do. In "3. Aa-Shuu Dekei-Oo" the vocal "mouth music" is an interlude to a song in the familiar "call and response" format. "8. Borbanngadyr" is the most spectacular example of vocal gymnastics I have found. Again, you are hearing one person's voice only; no instruments and no overdubbing!