Solaire linked the fire in ds1, so it was obvious who needed to voice the soul of cinder in dark souls 3.

Fun Fact: Miyazaki once stated in an interview that should you rescue solaire from the chaos bugs, he later goes on to link the flame, it's theorized that among all the linking ceremonies that took place in dark souls canon, his was among the first and was the only truly canon linking before dsII, meaning that the soul of cinder is, in essence, Solaire of Astora.