Hello. I'm no voice actor, but I've hired some of them in the past. Here are some tips:
-Use
https://www.castingcall.club/
It's a nice page to host castings for voiceover. Lots of people with decent mics and a will to work for free.
-If you're gonna cast someone, you should show that you are serious; that they aren't working for nothing (as in: noone will see their work). You should set up a
ModDb page for your Custom Story and present it as good as you can.
I'm pointing this out because at the time of writing this post there's no Amnesia mod called "Into the Abyss" on ModDb, and as you probably know, it's the best place to post Custom Stories.
-This forum is pretty much dead. For faster feedback on modding, you should join the Frictional Games' Discord server:
https://discord.gg/zPNPxa
(Keep in mind: this link will work only for 24 hours. PM me if you missed it)
Hope your mod goes along well.
Cheers!