Maybe he's just imagining things. Maybe he's going insane. Or, perhaps, it's normal to feel like there's a hidden sound in your life.
He's only started to hear it within the last few months, whenever he begins to utilize the Force without guarding his emotions. It's a dark melody, a fearsome march, and he knows that he'll hear it outside of his head one day. He's tried explaining it to Padmé, and even to Obi-Wan, but neither of them truly understands. Padmé just thinks that he wants to be famous enough to have music written for him, and Obi-Wan insists that the whole thing is all in his head.
All the same, it's there. It's there as his mother dies in his arms, as he slaughtered the raider-slaver settlement that had taken her. It's there as he decapitates Dooku, and it arrives again again when he saves Palpatine before allowing Mace Windu to die. When he sacks the Jedi Temple, the theme seems to have no ending; it just repeats itself in a loop, ad infinitum, and he never grows tired of it. On Mustafar, the song plays itself again. He battles with Obi-Wan, and the music only fades when his mind turns to black.
When he sees his son for the first time, it plays. As he fights Obi-Wan for the second time, it roars in his ears. When he tries to save the first Death Star, as he fights on Hoth, and when Luke's hand is taken from him, the tune is there. When he takes his son to the Emperor, he can hear it. As he throws Palpatine down, to his doom, the music only darkens in tone; though he is dying, he is temporarily the leader of the entire galaxy, and the players seems to know.
As he lays dying, his son Luke still before him, the music does not let up. It is not the same as Padmé's sweet voice, but it comforts him all the same. It is his song, after all, his own personal theme. Though the light leaves his eyes, and he returns to the light in his death, the wordless battle hymn plays without end.
You all know how it goes.