Mine didn't work, but I went into the setting of Google Now and turned off the voice and then turned it back on. After that, a notification appeared telling me to retrain my "Ok Google". I did that and now it works again for me from any screen.
It should be there
Settings - System - Language and Input - Voice Input - Enhanced Google Services - "OK Google" Detection
Sent from my Samsung Galaxy S6!
I'm on AT&T and the above is still fully functional for me as I type this message. As long as I'm not actively in an app - meaning I'm on any page of my desktop, not just the "home" page or in the google app, or on a page w/ the google search bar - then all I have to do is say "ok google" and it activates and awaits my voice command.
Unless it used to work even if you were actively in an app, then I don't know what everyone is complaining about.
The ability to use it from any home screen is what is gone. We no longer have that option under the OK Google Detection settings. All that lives there now is "From the Google app". Nothing else. Check yours in a few days and my bet is the option will be gone. You're lucky that yours still works from any homescreen. Are you on the stock Touchwiz launcher?
I don't recall having to turn this on when I installed Nova Prime (I think it comes enabled by default, but really I'm not sure). I also don't know if this is making any real difference, or if it just allows the Nova launcher to interact w/ the "ok google" command in general - but relies on the system settings for how/when it works.
And now this from a Google Community Manager on the Google product forum:
On Nova's settings there is an option under "desktop" that allows you to place a 'persistent search bar' on every page. That, plus the hotword recognition together is probably why it is working for you on every page. It's not quite the same as working from any screen - essentially you are activating the Google search widget on each and every screen. That is one way to get partial function. I have it set that way myself for now.
Was definitely working for me on all screens before the update and now it only works on screens where I have the Google widget added.
AT&T s6
Unfortunately, that was a verbatim repost of what he/she posted 6 days ago in the same forum.