What is the significance of omitting the indefinite article?

Hola Rock Anthony

In this case I believe they are including or omitting the article just to make it sound nicer as a song, because it fits better in the sentence. In songs everything is "admitted". I believe the original song in Portuguese also does that, it includes some articles and others are omitted.  As a very general rule, when we use the article, we are thinking of the noun as a specific "something" while when we omit it we are referring to the noun as a more "general meaning" of that noun. 

As a very simple example I could say:

In the first example omitting the article, we are saying simply that we have "coins" - in general; In the second example, with article "las" this is referring to some specific coins -maybe some coins that where mentioned before; with article "unas" this is referring to "a few" -we are being a bit more specific.

I hope this helps.

Saludos

Inma