Long Way, at the start of the level you are told Ares is the God of War. You have to make your way to Troy dimension to find the whispering voice to answer his question with "Ares". Each of the eight dimensions has a Whispering voice with a specific answer.
or there is a quicker way....
In Berlin, the "who Am I?" voice is at
Before you make the final kill in this level, ensure you have leveled up your characters is Skara Brae. There are ways to level up beyond this quest, but you have to backtrack to different dimensions.
Next stop, Malefia and Tarjan.