I've had my S5 in a LifeProof since it came out of the box in the store.
I agree with Jeff's comments above- it does cut down a bit on volume, and I have to have it in just the right spot on my ear. Also I've had a couple comments saying that my voice sounds a bit muffled, but not sure I can blame that totally on the case. IMO if you put your phone in situations where it might get abused, it's the best case out there right now.
To the simpleton, proof does not matter once emotion takes hold of an issue.