I never see frum women doing this, presumably because of marit ayin. As far as the issue of modesty with singing, I can see how marit ayin would apply. But if the prohibition on a woman's singing in front of a man were instead an issue of lifnei iver--or if there were indeed, as some seem to suggest is possible, no prohibition on the woman to sing, but only on the man to listen--would marit ayin still apply? --Or is there another reason women shouldn't(?) mouth the words besides marit ayin?
יש ליזהר משמיעת קול זמר אשה.
One should be careful not to listen to the voice of a woman singing
אבל קול דבור' שרי Speaking is permitted to listen too.
There is no mention of Maris ayin about being forbidden to mouth and we don't make up maris ayin unless the chachamim tell us The Shulchan Aruch Yore Dea 87,3, Rema, Taz, Pri Chodosh and Creisi upleisi say that Marit haayin "Hovu delo lehosif alah"
So a woman can speak the words of a song, and that goes without saying if she mouths the words without a voice.