Make sure this is the correct part:
Que si me muero sea de amor
Y si me enamoro sea de vos
Y que de tu voz sea este corazón
Todos los días a Dios le pido
There's a lot of "ifs" to this stanza, and it makes sense to have this element of uncertainty in poems and songs centered around love, prayer, the future, etc. There is uncertainty, and where there's uncertainty, Spanish speakers use the subjunctive mood. If you want a more literal take on the lyrics, consider my rough translation here:
"If I die, may it be from love
And if I fall in love, may it be from you
And that of your voice may it be that heart.
Every day I ask God..."
Half of the lines here have an "if, may it be" structure. It's quite close to the "if, then" strucutre in grammar that we call a conditional statement. If this, then that. If I die, then may it be from love. In a conditional statement in language or in a logic class, we don't actually know if the "if" part actually happens. We just know what will happen if the "if" actually happens. Because of that uncertainty, we use the subjunctive mood. The mood also explains why "si" and "sea" pop up a lot. Si means "if," and "sea" is a third-person present subjunctive conjugation of "ser."
Here's an article on conditional statements if you're looking for a more developed article on this type of grammatical structure:
I hope this helps!