what song does buck hear at the end of chaoter two?

That would be the song of his ancestry: the song of his primal soul:

Thus, as token of what a puppet thing life is, the ancient song surged through him and he came into his own again; and he came because men had found a yellow metal in the North, and because Manuel was a gardener's helper whose wages did not lap over the needs of his wife and divers small copies of himself.