turk: Welcome to the T&TH boards.
You ask a good question, and I think the answer is yes, predators can and do have the effect of diminishing turkey vocalizations. I doubt it is a factor when a turkey encounters an occasional fox or 'yote, but if they're seeing them frequently they are on pins and needles as you describe. Some people think vocal turkeys make themselves vulnerable, and the tendency to gobble is bred out of them because the non-vocal turkeys survive. I don't think that's the case. I don't think many turkeys are non-vocal, and I think it would take many generations for the "gobbling genes" to be bred out of them.
I don't know the answer to the question you didn't ask -- how to cope with that difficult situation. But I do believe turkeys don't need to be vocal to hook up with one another. I've seen turkeys get together without calling enough to believe they will find each other anyway. My best advice (I'm sure others will have better advice) is to keep at it, study the turkeys, their habits, their patterns, and learn how to put yourself where they want to go. Don't call much, and keep your calling soft.
By doing that, you'll invest a lot more time in the game, and you won't be as successful as you'd like. But other than that, my advice is to find a different place to hunt. There can't be that many predators everywhere.
Good luck. I hope others chime in.
When [url="http://www.EverydayHunter.com"]"The Everyday Hunter"[/url] isn't hunting, he's thinking about hunting, talking about hunting, dreaming about hunting, writing about hunting, or wishing he were hunting.