Voice
No one in Hadestown had a voice except for Persephone and Hades himself, and with the Queen gone to bring summer, Hades' voice was all the workers heard.
His commands rang in their ears: keep your head low; build the wall; work hard. When a daring worker raised their head, trying to speak up, the others quickly reminded them of their boss's orders, and the dissenting voice faded.
It was dangerous to have a voice, especially when you didn't have a name, and Hades' rallies reminded them of all he provided. In clear, booming tones, he told the workers why they built the wall and what it kept out, and the implicit threat in his voice that, should they rebel, there would be much worse than endless work awaiting them.
When the occasional break came, a respite from everlasting overtime, Hades' voice still echoed in their minds. And when the workers took to the line again, amid the whir of machinery and ring of the pickaxes, his voice overpowered all else.