Hey there,
Sorry for the late response here, but I wanted to make sure we got this addressed.
Thank you for bringing this up. Google Nest Protect has a built-in microphone that lets it check its own speaker and horn during each automatic monthly Sound Check or manual Safety Checkup test, Nest Protect uses its speaker and horn to make a short sound, and then uses its microphone to listen for that sound. If the microphone hears the exact sound that it expects to hear, Nest Protect will know its speaker and horn are working the way they should, and it shows you that everything is working in the app. Sometimes it can't verify the voice if Nest Protect didn't hear the exact sound it expected. For example, there might have been a loud noise at the same time as the test, like a door slamming or a TV playing with the volume turned up. Once you confirm that there are no unnecessary sounds that can interrupt the test, you can run again a manual Safety Checkup with the Nest app, or by pressing the Nest button twice, and listen for Nest Protect to sound.
