They definitely know an agency or something. The chances of them accepting a fan submitted audio file are slim to none, personally I've never heard of a studio doing so. There's valid reasons for that; not being able to control sound quality, needing lip synchs, contracts, potential legal issues, etc.
I know that studios like Larian Studios (Divinity Original Sin 2) bought in a lot of voice actors - what I recon they did was register with a company that Larian can contact and they help find the right voices.
You can write Rogue Factor if you want (not Focus, it's the developer you want) and attach a sample of voice acting for instance.
It does beg the question of what sort of voice acting will be included. As we all must remember there was basically 1 speaker for Warbands and the Narrator. Besides that your characters really only grunted and cheered. Are there going to be voice lines for your individual gang members, or will it remain only through the unique characters?
And what do people want?
I can see an advantage and disadvantage to both. On the more voices side it makes the game feel more alive, with characters having more, well, character. On the other hand there's a few drawbacks, I'd say largely stemming from logistics. As the OP represents for Ganger voices to be immersive we're going to need a decently large selection of tones and lines.
Which leads to the advantage of not having Ganger Voices. If there's not enough of them then Ganger Personality decreases instead of increases. Kind of like Fallout 4 when all your characters end up sounding the same, the backstory you try to invent isn't very convincing, seeing as your ruthless villian sounds the sane as your kindearted crusader. In this the silent protagonist is rather an advantage for immersion. There's also a developmental advantage as it diminishes time required for things like lip synchs and voice related animations.
For me I would prefer it if they added voice lines for gangers(and maybe a workshop option to add more) as long as they go big and have enough to maintain the illusion of individuality among your crew.