The interface is documented here: Mainboard/Pinouts.md at main · FrameworkComputer/Mainboard · GitHub
Realistically, you would get better results with a USB DAC in an Expansion Card than by replacing the Audio Board. Something like this would probably be the best you can fit: https://www.esstech.com/wp-content/uploads/2021/04/ES9281A_80A_PRO-Product-Brief-v0.2.5.pdf