I just got my Note 5 a couple of days ago, I set up my voice prompt but every time I say "OK Google" the Google prompt opens but then says it can't access the microphone. Just so you know I'm using the Google now launcher. It that the problem?
I think I remember hearing some time ago that if some app is installed that has access to the microphone, it can kill Google voice. Have you installed any music, recording, or voice type apps recently?
You could also try running the phone in Safe Mode to see if it works. I don't know if Google voice will run in safe mode, but worth a try.