Turns out, it's KUGIMIYA RIE. THIS JUST MADE MY DAY.
Omg, Paku Romi asked her to do Xiao Mei's voice for a bit, and Rie made this "shaaaaa!" sound. XDDD I LAUGHED SO HARD.
I can't wait for the scenes where Al and Xiao Mei appear together. EEEEEEE. *__*
(I plan on making a longer FMA post later, but for now I'll just say that I must have watched the second half of episode 19 like...at least ten times already. SO GOOD. And it's been great fun seeing all the reactions from people who haven't read the manga.)