Basically, "The Lost Slayer" BTVS book series has Buffy make a few different choices in Buffy season four, which leads to an alternate universe taking place: a worse, apocalyptic one, and most everyone ends up dying in it, including Angel. Buffy is able to go back and stop herself from making those bad choices at the end of the series--so things probably mostly go normal S4 after that point--but since Buffy still remembers Angel having died at the end of the books (though she does wonder if she'll eventually stop remembering the now alternate timeline), in really having tried to save her, that's why she wants to hear his voice in the above segment.






