How comes already there are no firearm firing sound detectors
There are: http://www.shotspotter.com/
Though results are mixed at best: https://www.cctvcambridge.org/DoesShotSpotterWork
There are: http://www.shotspotter.com/
Though results are mixed at best: https://www.cctvcambridge.org/DoesShotSpotterWork
Create loads of dirt cheap shotspotters with https://nurdspace.nl/ESP8266 plant them in your neighborhood. Make them report to community site so false positives could be eliminated by considering only simultaneous detection from nearby sensors.
Invite cops to use the site.