Sorrow
It had been a long, long time since Orpheus felt true sorrow. He'd cried when his mother had given him up, but Mr. Hermes' comfort tempered the ache in his chest. He still had fond memories of her, and over the years the good outweighed the bad when he thought of the Muse who had blessed him with musical gifts.
There had been a quiet dissatisfaction with his life up top, before Eurydice, but working on his song put it out of mind, and when he met her it was like he'd never felt unhappiness at all. Their romance only lasted months, but to Orpheus it felt like a lifetime.
And then she was gone to Hadestown, and the crushing guilt brought fresh tears to his eyes. You did this, his inner voice accused, and he had to make it right and get her back.
But then Hades revealed that only couldn't Eurydice leave, she'd signed the contract, signed her life away, of her own free will—choosing eternal hard labor over their love, security over the freedom to sing, and death over Orpheus himself.
The workers' fists were nothing compared to the sorrow blooming in his chest, and when they were finished Orpheus picked up his lyre and sang.
His music grieved her loss with him.