User blog comment:Ohshizim15/Big Daddy's Return to normal after they leave Rapture?/@comment-1330314-20100903111554

Technically, Jack didn't become a Big Daddy completely, he just slapped on the armor; so Tenenbaum probably could've surgically reverted the voice modulator process.

As for Alpha Series, they're transformed differently than the production-line BDs (Bouncer, Rosie, Lancer...): instead of having their organs grafted onto their suit, they just wear it (like Jack). For example, Subject Delta was able to remove his helmet without a problem. As for the rest, they're pretty much stuck as they are.