Ash always blames it on our height difference, because he's tall and I'm pretty short (the top of my head is a bit below his shoulder, or a better way to picture our difference: when we're both shoe-less and I hug him, my face is directly on his torso/chest/pecs). And whenever we can't hear or understand what I'm saying, he just says it's because I'm short, not because I speak quietly XD
But when I can't understand him (especially outside), I always say his words were taken by the wind, because it's so windy up there xD
Although it happens that he'll tell me to speak up, but it's usually when I feel super sick or faint. I just wish he wouldn't act like it's a crime to speak quietly when you're feeling sick
Anyways, we usually make jokes about it.
And now that I think about it, I don't recall my parents or sister ever telling me I speak quietly. Usually it was teachers...