When I upgraded from my Google Pixel 3 to a Pixel 5, the Google Assistant Open-Mic Notification, AKA the Google Assistant Hotword Beep stopped working. Moreso, the instructions to turn it on using Switch Access didn’t work! Here is the fix!
Watch the video or follow instructions below.
Go to your Pixel 5’s Settings and look for “Accessibility: Accessibility” (don’t tap on the “Accessibility: Google Assistant” screen… yet!). Go to Switch Access. Turn on “Use Switch Access”, click to “Allow” it to have control of your device, click “X” in the upper right corner when it asks about choosing a switch type. At this point, the Mic-Open Notification should work but it might not! That was my case.
Go to Settings and search for “Accessibility: Google Assistant” and tap on it. You’ll be at a screen that says “Accessibility, Mic-open notification, Mic-close notification”. Both will probably be turned on. Turn both options off. Then turn them on again. That was the magic! Now your phone should make a bell sound whenever you say “Ok Google” or “Hey Google”.