Originally Posted by
Pau_GyG
Maybe I can give you some kind of explanation. The pronunciation of this sounds depends on the region/country where the singer/speaker comes from. For example, in Argentina most people pronounce "yo" with the /sh/ sound as in "shoe". And the same goes for the cluster "LL". In some other countries this sound is produced with the semi vowel /j/ (eg: the first sound in "year") in the case of "yo", or /l/ followed by the semi vowel /j/ in the case of the cluster "LL".
However, you can find different pronunciations of this sound within the same country. For example, as I said before, most people in Argentina use the /sh/ sound, but in the upper clases "Yo" and "eLLa" are pronounced with a similar sound to the first sounds in "genre". This pronunciation is frowned upon by common people, and it is used to make fun of rich people.
About the songs you heard where the pronunciation changed from one line to the other, I think itīs a matter of rhythm. Probably the pronunciation which is repeated most of the times is the one that the singer actually has.
So, basically, if you are learning Spanish pronunciation I think itīs a matter of choice... To what kind of native speaker are you aiming to sound like?
Hope I could help in some way!