[Motorola Droid RAZR] Ohmster:
That is odd, it should work. In your app drawer you should find Google Settings, as if it were an app. All Google settings can be set in this app. Make sure you have the search language installed. Under Voice is the OK Google detection to turn it on. Speech output, turn it on. Let me turn on mine just to make sure it actually is working in this build. I tried to use it and found the OK Google detection was more of a novelty then a useful application and turned it off. I wanted the extra batter life from not running my phone at a double idle frequency. Plus, it just was not actually "useful" in any meaningful sort of way. By the time I actually got what I wanted from OK Google, I could have had it manually, in almost every case, faster. But, let me see if it works. Of course your microphone must actually be working in order for this to work. If the app can hear you when you tap to activate, then it can hear you all the time, as long as it is activated and set properly.

Hmmm, doesn't seem to be working for me either. The phone still idles at 300MHz. :(

But there should be more to this. A place to do voice training and to set it to answer from all desktops. I am not seeing that either and OK Google is turned on, not working. These apps update so often that perhaps the app has changed. Maybe Privacy or Security settings need be checked. No, nothing here either. Clearly, the OK Google detection is not working. My phone is idling at 300MHz and not 600. I am really surprised at this, especially that the voice training and "use OK Google from any screen" cannot be found. I will do a real Google search to see if I am missing anything...

Yeah, there is something wrong with the Google app. I did a search on how to enable it and the Voice option in Google settings does not contain the old familiar "From any screen" box or the Retrain or Delete voice model anymore.

It actually does work with the Google app open on the desktop, but should work all the time.

Here is a post that shows how to get it working again:

How to install the Google Experience Launcher with working “Ok Google” hotword on your Android device

There is a download for Google Search 3.1 & Google Home (GEL)

Google Search is going to update itself unless you deny it and Google Home is no longer installed. I do not even have a Home setting in my Settings anymore, previously I had Apex, Google, or Trebuchet. You might actually need Google Home to get this to work. I will install the Google Now Launcher to see if this is the problem, but I want to keep the Trebuchet desktop. Yes it is possible you need this, I believe I had it installed before when it did work. Well, I got the Home button back again since there is more than one launcher installed. OK Google still does not work, no retrain or delete voice model options, nor detection from any screen. I think when you enable this, you get a disclaimer to agree to about location tracking, not sure, but there were disclaimers to acknowledge before it would turn on. I am not getting the disclaimers anymore.

Ugh, to even try this Google Launcher, I have to clear all of my defaults and enable it. I really do not want to do all of this because I don't like or want the Google desktop. Been at this for close to an hour and when I don't get anywhere, trying for too long, I stop. Give it a rest. Not sure why it isn't working but at least your complaint is confirmed. We might get other answers from users who use the Google desktop to see if that is the issue. I also found a bunch of app conflicts with OK Google search and the list is too long to print, plus it can be anything. Let's see what others have to say. :(

EDIT:

I think the Google app has changed. Clearly, there is a difference in what the OK Google used to say and now what it does say. Screenshots attached of my screen, v.s. another screen of what it used to say, "Say OK Google to launch search" before, now "Say OK Google to start a voice search from the Google app". Those two sentences are quite a bit different. One of them indicates a search when you say it, the other specifies you must first launch the Google app and it does work when I use the Google app. I think that Google "took it out". Attached are the two images. The one in green is the old OK Google screen, the one in blue is what I have on my phone now that no longer works from speech alone without tapping the Google app. They are different.