"a" vs "o/u"

[idiez at MAC.COM]

I have a pronunciation question regarding the nahuatl of northern 
Veracruz.
Here is the present tense conjugation of the verb "to go"
        niyauh, I go
        tiyauh, you go
        yohui, he, she, it goes
        tiyohuì, we go
        inyohuì, you (pl) go
        yohuì, they go
I have always written the "yohui" with an "o", because it is pronounced 
as a "u". And I have always wondered why it isn't pronounced "yahui".
During my most recent visit to Tepecxitla, Veracruz, I encountered the 
following:
        ayohuitl, fog (the "o" is pronounced as a "u")
        tlaayauhtoc, there is fog.
I should also mention that the syllable final "-uh" is always 
pronounced as an aspiration (example: niccahua > niccauhqui, etc.) . 
There is no rounding of the lips.
I would seem that a syllable initial "hu-" (with its rounding) is 
indeed changing the pronunciation of a preceeding "a". If this is the 
case, I can go ahead and respell all forms of "fog" and "to go" (and 
other instances of this situation) with "a"s, just like you find in 
Molina.
However, it this is the case, "cahua" would be pronounced "cohua", 
unless there is something about the long vowel of "ca:hua" which would 
interrupt the process.
Any ideas?
John

John Sullivan, Ph.D.
Profesor de lengua y cultura nahua
Centro de Estudios Prospectivos
Universidad Autónoma de Zacatecas
Director
Instituto de Docencia e Investigación Etnológica de Zacatecas, A.C.
Francisco García Salinas 604
Colonia CNOP
Zacatecas, Zac. 98053
México
Oficina: +52 (492) 768-6048
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idiez at mac.com
www.idiez.org.mx