Barillo's piano teacher explains to him that if one's soul is pure, music flows clearly from it. And if it isn't? You have to practice a shit ton. One can infer from El Mariachi's beautiful guitar playing that his soul is either very pure... or he just practices all the time.
Which would accurately describe an evil person trying to do good. Such as Agent Sands, murderer and agent provocateur, playing his part to foil a coup and assassination attempt on the President's life (admittely a plot he had his part in creating, and admittedly out of revenge for what Barillo and Ajedrez did to him.)
It can also be taken to mean that nobody has a pure soul, as even the most talented musicians typically practice quite a bit.
We know from Sands' conversations on the phones that his contact was unwilling or unable to get the weapons that his people would need to stop Marquez's coup. So where do the townspeople get all that heavy weaponry from? Either they were able to get that kind of firepower on their own, or else Sands' contact came through for him after all.
