The world may never know.
In all seriousness, though, I believe Mila is the only English voice we don't know the name of. Some people said she was voiced by Lauren Landa for a while, sharing a voice with Kasumi, though that was rather iffy considering the questions it brought up (most prominently, why could we confirm she was Kasumi, but not Mila?). Another popular guess was Christine Marie Cabanos, though she was later confirmed to have no involvement with the game (this was before Marie Rose came along, who actually was voiced by Cabanos). A somewhat lesser known guess was that she's voiced by Cristina Vee, who I don't think has had any confirmation either way, and her voice does seem like it could be Mila. That said, I believe Vee typically announces the roles she gets, so it seems strange that we wouldn't know if it is indeed her.
Unfortunately, it's likely she was voiced by a lesser-known union actress, and DoA5 was a non-union production, so the actress revealing her involvement could be potentially damaging to her career. As such, it really is unlikely we'll ever know.