Xalatath and Nyalotha

[formerShandalay]

I don't think the voice gives anything away. I mean, Yoggi was Sara and had a female voice... unless he whispered or screamed ^^

[Varodoc]

Well Blizzard acknowledged that players call her Knaifu and even renamed it "Bae blade" in a joke patch note. And since this is Blizzard and they often embrace the memes in the story, it wouldn't surprise me if Xal'atath is indeed female.

[Minikin]

I don't think the voice gives anything away. I mean, Yoggi was Sara and had a female voice... unless he whispered or screamed ^^

That fight was so epic. I loved how the music changed in the last phase with the immortal guardians. Like the hurried pace.

[roflolmao]

The old wielder of Xalatath knew about the existence of the blade by studying void wielding orcs during the first war. Orcs used the blade in rituals, meaning the burning legion was ok with using the blade, maybe the orcs/burning legion captured an old god somehow and used it like our warlocks capture demons and command them.

[Raisei]

Well Blizzard acknowledged that players call her Knaifu and even renamed it "Bae blade" in a joke patch note. And since this is Blizzard and they often embrace the memes in the story, it wouldn't surprise me if Xal'atath is indeed female.

This is very likely closest to the truth.

Though @Aucald is probably correct that the Old Gods have no biological gender to speak of. But I assume that it will work much like with Hutts and Droids in Star Wars, while neither male nor female biologically they adopt/are programmed with a behaviour and voice pattern that fits one of the genders more or less.
So Xala'tath might be female because it chose to be, but as the story goes it might also be coincidence since it's previous wielder and the blood elf she posesses are both female.