I see. Very well, let's compare Female MU x Chrom, then. I'll only put the S support because that's the only one available for them (there are few "mute" conversations in the site I use)
So yeah, it doesn't change much from "Watashi" to "Atashi". The former sounds more polite, while the second sounds a little childish. But for "mute", Chrom says not only his own dialogue, but snipets of a voiced MU.
...suddenly mute MU sounds very interesting.