Oh Google, all the mysterious ways in which you work will continue to elude us, but now we shall shed the light on one very important puzzle that's been making us scratch our heads for months, nay years. One day you say "OK Google" to your phone and it responds with a ding letting you know it's listening. A few weeks later, you try again, and there's no ding so you have to look at your display and spot that tiny animation on the bottom to figure out if it's listening to you or not. What's the difference? It's not your phone's volume level - you sometimes hear the ding even with Do Not Disturb on and then other times you don't hear it with the volume up to maximum.
Well, it looks like the difference lies in one half-logical, but fully silly, change: Accessibility services. If all of the accessibility services on your device are set to off, you'll get a vibration but you won't get a ding. Try as you might, you'll yell, you'll raise the volume of everything from the ringer to the notifications to the media, you'll shout with your screen on or off, to no avail. No ding shall be dung for you poor thing.
But then change one accessibility service to on and that will wring the ding from the depth of your phone's root files then fling and sling it at your ears with the thump of a thousand pings and tings. How transcending!
I have spent the past hour enabling then disabling the Nova Launcher accessibility service and it works every single time. Service off: no ding. Service on: ding. Service off: no ding. Service on: ding. Dumbfounding. But not the first time we've seen Accessibility modify a feature's behavior.
The ding will be heard each time you issue the OK Google command, even if you're already talking to Assistant, it'll work regardless of whether your phone's volume is high, on vibrate only, or in Do Not Disturb (alarms or priority modes). The only time you won't hear the ding is if your device is in DND but set to Total Silence. Oh and keep in mind that the ding follows the voice command "OK Google" and won't be heard if you press and hold the home button for Assistant. (In that case, I guess you're already holding the phone so it doesn't matter a lot.)
Now that we've solved this confounding mystery, I'll leave you to test it out and find the best -ding word to describe your sentiment about this astounding discovery.
Alternate title: Ding ding ding! We have a winner for the mystery of the missing OK Google ding!