Hi, guys. This is something happened to several other people.
I can activate the voice command indicator but it won't hear at all.
Voice commands/Google Assistant on the controller instead just work fine.
I tried to fix it erasing the Google app cache or connecting it to usb port, as someone suggested, but it won't work. I also enabled the 'always on' feature on my remote.
Occasionally, it still works, but it's a 99% fail.
Tell me that someone found a solution, please
I bought my 2015 Shield TV second hand for $100 about 18 months ago and the remote (which the seller claimed they didn't use) had the same issue as yours. (Although mine never worked ...unless I plugged earbuds with a microphone into it)
I sent an email to Nvidia support asking for tips to try and solve it and they ended up replacing it for free for me. I just had to pay shipping to get the old one to them. Might be worth a try. I was surprised by their great support...especially since I wasn't the original owner.