Last week, the New York Times wrote about a gunshot detection technology called ShotSpotter.  Similar to video camera monitoring technology, audio detection devices are set up throughout a city.  These devices pick up the sound of a gunshot and triangulate the location.  Back at a control room, a technician notifies the police.   Within minutes of the initial gunshot, police arrive on the scene often before a 911 call is even placed.
According to the Times:

Many police officials say the system has significantly improved response time for crimes involving firearms and has increased community confidence and helped deter gun crime by demonstrating that the police can show up quickly at the right place.

The technology, they say, has given officers critical information about what to expect upon arriving at a crime scene — like whether a gun was fired from a car and if so, how fast and in what direction the car was traveling — and has offered a level of precision in locating gunfire rarely afforded by 911 calls.

As long as the audio devices only listen and record gunshots, it’s a promising tool for law enforcement to improve their response times to gun crimes and their chances of apprehending the perpetrators.
The downside is when the microphones start recording private conversations.  As the Times article points out, they already are doing this in New Bedford, Massachusetts. 
Jay Stanley, Senior Policy Analyst at the ACLU Speech, Privacy and Technology Project weighs in with the ACLU’s view:

It is not generally legal for law enforcement (or anyone else) to make audio recordings of conversations in which they are not a participant without a warrant. In addition to the apparently accidental eavesdropping reported by the Times, an important question is whether microphones can be remotely activated by police who want to listen to nearby conversations.

We’ll be watching closely to ensure that these systems don’t become the latest way for police to get around the Fourth Amendment. In New Bedford, the district attorney told the Times he thought the recording could be admissible as evidence. If the courts start allowing recordings of conversations picked up by these devices to be admitted as evidence, then it will provide an additional incentive to the police to install microphones in our public spaces, over and above what is justified by the level of effectiveness the technology proves to have in pinpointing gun shots. It will also reduce the incentive of the manufacturers to design these systems so that they only pick up gunshots and not the conversations of people walking down the street.