"Beth dych chi'n gallu wneud yfory?"
Translation:What are you able to do tomorrow?
I think it's the beth at the beginning that does it, or rather an ei which "repeats" the beth and which causes the mutation even after it dropped out of common speech.
So this was originally, I believe, something like Beth dych chi'n gallu ei wneud yfory? (literally, something like "what are you able to do it tomorrow?").