In the latest version of the Google App, which is v7.4, there is now a bug that Android Auto users are noticing which prevents the "OK Google" voice command from working in the car. Considering that this is how you interact with Android Auto, for the most part, this is actually a pretty big deal. Many Android Auto users have taken to the Google Product Forums to raise the issue, and Google has stated that it is aware of the issue and are working on a fix. It's currently unclear when that fix will be available, but Google did give users a temporary workaround to fix the issue.
The workaround is to turn off the "OK Google" hotword detection from any screen. This is done by going into the Google app, tapping on settings then Voice. From there users will see an "OK Google detection" section where they are able to turn it off. This does allow users to still use OK Google in the car, it just turns it off from their smartphone, so it means that their smartphone isn't always listening anymore. Which might be a good thing. There are some other fixes floating around in the Google Product Forums, but most of them are not reliable and only working for some users. But this workaround from the Android Auto team does seem to be working for everyone, which is good to see.
Users should expect to see an update coming for the Google App in the very near future with a fix for this issue. But for now, go ahead and use this workaround, if you do use Android Auto. It's a bit odd that Google didn't test that out before rolling out the update - but then again Google did essentially roll out an update that broke Play Music on Android earlier this week, so it's not that surprising. If you are experiencing this issue, be sure to head to the Google Product Forums and make your voice heard so that Google knows it is affecting a large amount of people and it gets fixed as soon as possible.