I also feel that this is the Master's lament and repentance for his crimes and trangressions against not only the Doctor but also against all civilizations he has destroyed, altered and set into descent. But the Doctor being the Doctor, he forgives the Master and is willing to help him live and become peaceful, but the drums will always torment the Master and he realizes this and that is why he chooses to not regenerate.
I also feel that this is the Master's lament and repentance for his crimes and trangressions against not only the Doctor but also against all civilizations he has destroyed, altered and set into descent. But the Doctor being the Doctor, he forgives the Master and is willing to help him live and become peaceful, but the drums will always torment the Master and he realizes this and that is why he chooses to not regenerate.