I've seen it on t.v. Its like a type of fast rythmic chanting thatt repeats "Cha" over and over. I thought this was in Thailand but I cannot find it or what this type of music is called.
I've been looking around youtube.com for Thai chanting and there are plenty variations but nothing with a rapid cha over and over. This would be a good place for you to look for your mystery chant. I guess you will have to try different countries and cultures - I did not search under Hmong chanting. Sorry I could not help you.
