In chapter 20 of the lightning theif, why does percy suspect that ares wasn't acting alone and that he was taking orders from someone?

If music be the food of love, play on; give me excess of it, that, surfeiting, the appetite may sicken, and so die. that strain again! it had a dying fall: o, it came o'er my ear like the sweet sound, that breathes upon a bank of violets, stealing and giving odour! now read the excerpt from "the love song of j. alfred prufrock." for i have known them all already, known them all: have known the evenings, mornings, afternoons, i have measured out my life with coffee spoons; i know the voices dying with a dying fall beneath the music from a farther room. what does the phrase "dying fall" most likely mean in both excerpts