Daniel Bowling really ought to do this study with Vietnamese speakers. In Vietnamese, there is a long tradition of interaction between the six tones of the spoken language & the (mostly pentatonic) music used to accompany songs & poems. Last weekend, I was invited to the home of a family of very talented traditional musicians & treated to an informal concert of various forms of Vietnamese music. In most of these forms, because the language itself is tonal, the poet or songwriter has followed forms that place rising or falling or level tones in particular places in the verse line. The instrumentalist(s) & vocalist, also following a set of conventions, but also improvising within the conventions, perform the text so as to emphasize & play with the composer’s intentions. My sense is, though I’m not entirely sure of this, that, contrary to the Western art song / lieder tradition, the music is driven by the vocal text rather than the other way around.
The musicians I listened to played the đàn tranh & the đàn đáy, stringed instruments with a pentatonic tuning & the ability to bend notes in order to suggest the tonal slides & glides of the Vietnamese language. There is also a two-stringed lute that has no natural tuning & can be tuned to any of the other instruments. Half-way through the concert the other night, a young man showed up who was a virtuoso on this instrument, playing incredibly complex runs of notes on the raised frets. In the south, I have seen Western guitars with the spaces between the frets scooped out in order to play microtones.