I know this fear intimately <3 :( It's distressing how fallible our brains are, especially when we are capable of so much love—and of love that endures even death. I hope for you that you never forget, and I also hope that this might be comforting: Even if we lose the ability to recall clearly the sound of a loved one's voice (i.e. the dendrites related to that specific memory die off), that memory will still be there, albeit more difficult to access. But its being there means if we were to hear that voice again, even if in some messy recording somehow, we would recognise it quickly as familiar and important and, very likely, if not right away then soon after, as belonging to that beloved person. And it would make other related memories easier to access too.
I know that can seem like a lofty hope, a pipedream, a slippery bean chase: if I could just find a recording somewhere, etc. And sometimes we're certain, too, that there is none to be had, and that hurts. If nothing else, the feeling of love in us for our loved ones that is so often (re)inspired by the sound of our loved ones' voices stays with us. The heart remembers, even when our mental ears cannot hear anymore.
I wish for you wellness and more of the love that makes voices so precious and an impeccable recall for them (or else freedom from painful self-doubt), and I hope you're doing alright out there.