In the song "Cielito Lindo", canta y is pronounced as if it were cantay. Is this standard, just a singing oddity, or a regional variation?
In Spanish poetry, when counting the syllables in a verse you must count as one syllable when the last syllable of a word ends with a vowel and the first one of the next one starts also with a vowel. This is knows as sinalefa:
Del lat. tardío synaloepha, y este del gr. συναλοιφή synaloiphḗ, der. de συναλείφειν synaleíphein 'confundir, mezclar'.
- f. Fon. y Métr. Unión en una única sílaba de dos o más vocales contiguas pertenecientes a palabras distintas; p. ej., mu-tuoin-te-rés por mu-tuo-in-te-rés.
To give you an odd example:
Me gusta el arte abstracto.
Me-gus-tael-ar-teabs-trac-to. (7 syllables in the verse.)
This is the general rule, as this is how we pronounce in real life, linking sounds. There are exceptions but in your case it is just like this:
Canta y no llores.
Can-tay-no-llo-res. (5 syllables in the verse.)
When singing, you need to sing according to the syllables in the verse, that's why you need to apply the same rules. In fact, you have another example in the same song:
Porque cantando se alegran,
cielito lindo, los corazones.
Por-que-can-tan-do-sea-le-gran. (8 syllables in the verse.)
Note that once more I have grouped
se alegran. This linking thing is not exclusive to the Spanish language. Just think how the English language does the same when rendering
he is into
he's. It is the same principle, only that Spanish does not contract words (with the exceptions of
del) in the written expression.