From the Doom wiki, when we first pick up the Soul Cube, it tells us:
Destroying evil gives us life, and makes us stronger. Unleash us when you hear our call. We will pass the life force of those we slay to you. We foresee a great battle, but the outcome is not clear. We are the only way to destroy Hell's mightiest warrior.
And every time it is charged we hear:
What is the in-universe explanation for how the Soul Cube is actually triggered? How does the Doom Guy communicate to the Soul Cube he wants it to activate?