Like everyone has already said, the Maiden of the Mist is an assimilation of souls bound to some kind of spooky form and Riot has pretty much confirmed this to be the case.
But like all cheeky skeptics I say BULL SHIT RITO! If the Maiden was made of tormented souls and in no way a single entity then wouldn't it be cool make sense that it's VO would be a conglomerate of voices? Something both female and male in nature, all speaking in unison and hell that would be creepy too. But the evidence we have says no, the Maiden vocalizes with a single female voice. So if something is meant to be everyone and yet no one at the same time then what is the importance of making her VO female?
Hence why logic would say that even if the Maiden wasn't specifically the Ruined Queen, she is at the very least a huge part of it's embodiment.
But if Riot says that's not the case, then i guess that's not the case. At the end of the day, the Maiden's VO could just be a wink and nudge to the Ruined Queen, nothing more nothing less.