A woman should not sing along to a song in the presence of unrelated Jewish men. But can she mouth the words, or sing to herself?
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?