Namine speaking to Roxas then makes absolutely no sense. The voice spoke of Memories of "the both of us", and "being a part of you [=Roxas] just as you are a part of Sora". Despite Namine coming from Sora, she returned to Kairi, while both Roxas and Xion are inside Sora. Also, at that time, Roxas and Namine didn't even meet :\ Roxas didn't even know Namine existed. So why do so many people think it makes sense that voice was Namine?
I meant when "Xion" told Riku to stop Roxas. Wasn't THAT Namine?