Originally Posted by Perikles
I don't really understand the thrust of this statement. Generally, in English, an unstressed vowel is usually weaker than when stressed, and is often reduced to a schwa. But it doesn't change to another vowel. Is this what you mean?
Exactly, that's what I'm talking about from the very beginning. Schwa is a different vowel. A weaker unstressed vowel is also a different vowel (/i:/ is not the same as /ɪ/ and certainly different than /ə/).
Every standard in English has built-in rules about that. Spanish doesn't. Only marginal ways of speaking have such fluent changes and twists in pronunciation, generally related to poverty, or in contact with native languages, foreign languages or inherited characteristics from the original languages of slaves. That song falls into that category, as well as many people from Canary Islands have some nuanced speaking that, like Andalusian, reminds of caló
Educated speakers in Spanish use lots of vocalic sounds only when we speak with our mouths full.