Both my 1st Gen. Moto G, running Lollipop, and my 3rd Gen., running Marshmallow, had/have the acknowledgement tone sometimes work, sometimes not. And, when the acknowledgement tone doesn't work, I also don't get the voice confirmation on voice dials.
Have never been able to determine what action or sequence-of-events made/makes them stop, or what makes them mysteriously work again. Makes the whole "Google Now" thing rather kinda useless, for my purposes. I rarely need to know where's the closest sushi bar, but, I do often need to place a call while driving down the road and paying attention to, you know, driving.
As for reporting it to Google: Yeah... I don't think I'll waste my time on that exercise again. I'd found that, after pairing my 1st Gen. Moto G with my ASUS ZenWatch2, that my phone's notification tones would mysteriously go away entirely--requiring a reboot to get them back. Somebody else, using a completely different phone and version of Android, discovered what was triggering the behaviour: Using Google Now to voice dial from your watch. I gave Android Wear a less-than-stellar rating for that, Google reached out, I followed their suggestions. Re-did everything they suggested, including painstakingly FR'ing and re-pairing the watch with the phone. No joy. Their CSR eventually simply never followed-up on the results of the last thing he had me try. IOW: When the "Did you try rebooting it?" (MS-Windows, anybody?) did not work: They simply gave up.
Same thing happens with the new phone, btw. My conclusion: Google Now is just plain crap software. Poorly designed, poorly written, and poorly tested.
I don't use it, much.
Good job, Google!