People, get with it: Kevin is gone, and whoever takes over Elmo, they're not going to sound 100% exactly like Kevin...
There's a very subtle glottal thing, but as I've said before, getting the personality down trumps sounding like the character. Ryan's doing a great job as Elmo, and it seems that they're not going to scale him back once they run out of Kevin appearances that were taped well in advance. Elmo is too important a character for Sesame Street to let go of, and there was no doubt an understudy considered if something happened.
Somehow, it seems that across the board, they're more willing to recast characters in the event of a death/departure. It took how long to get Ernie back on Sesame Street? Not to mention Guy Smiley, and that was a one shot appearance. Since Matt's been Jerry's copuppeter, the recasts of his characters were smooth and quick.