"Salut les garçons !"
Because it is not greetings some boys but rather greetings the boys which is the boys who are present. We know he is not saying greetings boys in the same way as he might say hello world. We know he is addressing some particular group of boys because he says salut les garçons.
In English we don't have to attach a modifier to the noun so we leave it to the listener to figure out whether he is addressing all the boys in the universe or simply those who can hear him. In French there must be a modifier so they use the appropriate one.
And you will never be able to hear that "s" because it isn't pronounced. In french, the final letters are skipped except in particular cases where the next word begins with a vowel and then the two words are linked with the final letter. Check this article about liason (word linking)
In this case, there is no liason, so the only difference lies in how she pronounces the "le" and "les". Check the pronunciation here: