I've heard a few songs (off the top of my head, A Dios Le Pido) where they pronounce "y" both with a y-sound and an English j-sound at times. In some songs, the different pronunciations occur when repeating the same phrase (from the song above, "yo a Dios le pido").
Is it just something that can occur in music? To my knowledge, people add accents in English music, but for any one song they stick with one accent. Is this not the case in Spanish?
When it comes to songs, everything and anything is valid in Spanish. Just listen to 5 minutes of reggaeton and you'll see how every possible grammar and pronunciation rule is broken.
Singers usually do this sort of thing to force a rhyme, for example.
This can happen in normal speech too. For many speakers of Spanish, the y will be pronounced as something better described as halfway between the English y and j. That's a rather inexact position, and you try to do it you'll notice sometimes it comes out as more of a y or more of a j. Depending on phonetic context, stress, intonation, etc, the Spanish y may end up adopting pronunciations closer to the English y or the English j. Musicality becomes just another variable in the equation.
(I avoided IPA here to avoid possible confusion since English y is [j] in IPA.)
It can depend on who is doing the singing, and on regional accents, too. In Argentina, the "y" sound is very close to the English "j". If a Mexican is singing an Argentine tango, sometimes he will drift in and out of imitating the Argentine accent.
The same thing happened during the "British invasion". Some Brit pop singers who spoke with their own regional accent would try to sing as if they were Americans. The imitation was usually very convincing, but occasionally, they would slip up.
