I have an issue with that voice, and that is... well, it totally doesn't sound quite right, true, we have no proper idea of what Kasumi should sound like, however for most characters in game settings, you can look at the character, and try to picture a voice, like with Zaeed for example, without ever hearing him, you could look at a picture of him, and think, hrmm, he looks like a battle hardened older guy, probably a rough deeper voice, but he doesn't look as if he'll be 100% serious sounding ALL the time either. And that's a kinda proper description.. Tali, you can imagine her having an accent, perhaps a mechanical tone of some sort to it, and yeah that fits. Thane you can look at and think, well perhaps he has a bit of a fishy voice, he looks kinda like a fish, oh, he's a lizard? Interesting, looks more like a fish.. and well, his voice does have a watery, gargling tone to it... My point is, that those files don't even seem... well, they don't seem complete, they could simply be a woman who works for Bioware, who, if they came up with Kasumi as a joke, they just had her do a few lines.
ALSO Explain to me, why would a DLC character be using audio files that were added during regular installation? It would make more sense for a DLC character to be using audio files that were downloaded during that character's DLC download process, no? Those files also seem to only be "idle chatter" type audio files, meaning the things she would say if she were on the Citadel, or Illium, or Tuchanka, or Omega, you know how there's the "Talk to Garrus" type thing that pops up in the hub worlds, those are what I'm talking about. It's not as if you've got audio files that would play when you meet her, talking about her story, and how she joins you etc...
Also, one big consideration that just came to my mind when listening to those... How do we know she'd really be a character that would join you? For all we know, she could simply be a character who is involved in a line of city missions that is involved in a future DLC. Example: It starts in the Citadel, Captain Bailey mentions to you that there has been a bit of a problem lately with a thief (Kasumi) who has been stealing things (he'll list a number of things), and Shepard takes it upon themselves to go find out who's responsible, within that, they end up finding Kasumi, who they end up having to chase, from the Citadel, say she goes to Illium, and the audio files that have been found, they can simply be a small thing like if you chat to her before that part of the mission is active, she'll give you some flavor chat. Anyways, you then go to Illium, chase her more, she escapes and makes her way to Omega. There's no necessity that Kasumi would join you at the end of this mission line, it could simply be a mission such as this... True that it could be considered a bit of a stretch saying all of this, however, it's no more of a stretch than people saying she's ABSOLUTELY a future DLC character that joins your squad.