I have set up nest aware. I was on chat with support for a few hours. My Google Home Mini is setup with my nest aware. I played a smoke alarm sound on YouTube and nothing happened. I noticed my Google home mini can only hear after I have tapped on it. My Google home mini will not hear anything as it's idle. I'm trying to fix this issue so that if an alarm is going off I can hear it when I'm not home - when there's no one there to tap on it for it to hear anything.
I had the same issue, smoke alarms not being detected on any of my home devices (mini's, displays, wifi points) despite being enabled in the settings. I just tested it with a youtube video of a smoke alarm and it finally worked. So it appears it really needs a prolonged sound to work, and the device it picked it up on was a Nest Hub display, not the mini's that I was right next to, so that may be the problem for you. Sorry.