Curiosity: foley tropes

[elainegrey]

We were watching an X-Files (OMG, actors so young, hair so fluffy, mobile phones so clunky) last night and a little atonal two note flute motif played to hint to the viewer "Vietnam veteran." (Christine isn't sure if it's a minor second or perhaps an augmented fourth, not in a major key. She says there's a flutter involved as well.) I swear i've heard that same little motif used before to indicate "Vietnam!"

I presume it is the flute Wikipedia describes as

The sáo (also called sáo trúc or "sow trook", pronounced [ʂǎːw ʈʂǔkp]) is a small flute found in Vietnam that is traditionally thought to contain the culture and spirit of Vietnam's countryside.

That explains the instrument but not the two note motif. I assume the particluar interval between notes is common to Vietnamese music, but why do i know that? How did this auditory shorthand get to American culture so that i can hear two notes and know "Ah, this has to do with Vietnam"?