A woman would not launder clothing outside a beis medrash, in a marketplace or somewhere where men congregate so it is the man’s fault if he walks into her backyard while she is doing the laundry and see’s her uncovered arms. So too it is his fault if he walks into the local girls’ high-school production and hears girls singing. On the internet however which can be likened to the marketplace, nc is putting herself in a place where men can easily ‘pass by’. Therefore it is her fault.
One point that seems to have been overlooked is that by nc putting her songs on youtube she is being oiver on lifne iver lo siten michshal. No matter if the person listening would have listened to other female voices anyway, by them listening to HER voice she is causing them to do an aveirah (which is an aveirah in and of itself).