Speaker setup/quality probably also helps.Someone mentioned it, but a lot has to do with their mic setup, I think. I can't understand a thing they say on speakers, but sound totally fine on headphones. I had the same issue with Mumei for the longest time until she moved and got a better setup. I think it is just exasperated with FWMC because there are two of them.
I have usually no problems understanding them even without headphones.