I was having the same issue with S-Voice on my Samsung Galaxy S4 all of a sudden. A Samsung support tech helped me figure it out by telling me to start the phone in Safe Mode to see if S-Voice worked that way.  He said if it did, it was most likely a 3rd party app.  Turns out he was right.  I deleted Music Download Paradise Pro, which was the most recent app I had downloaded and S-Voice has worked since then.