Dunno if it's been said already, but from what I can piece together, I'm thinking that Cullen kissed Bay's ass a little bit and got the job.
From reading interviews with him, I really see him as that kinda sort.
I think maybe Welker didn't kiss ass, and Bay being such a fucking insecure individual decided not to use him because of that and instead used Hugo and altered his voice to sound like Welker. Just as a fuck you.