On modern Androids (6.0+) accessing the microphone requires an explicit permission granted by user (just like on iOS). Furthermore, due to Doze mode implemented in 6.0 and expanded in 7.0 running these kind of services without notifying the user cleary (with permanent notification) has become pretty much impossible and unreliable. To run in background when the phone isn't in active use the service needs to be marked "Foreground" which demands a permanent notification.
Having said that - Google is failing here in several ways. First, they STILL let apps be published that don't target Android 6.0. Apps that target older Androids get permissions granted at install time (those can be revoked, but user needs to go to settings for that). There's no reason to let people publish new apps that don't adhere to new permission model, but Google just doesn't care.
They also need to clearly mark use of system resources in the background - just like we have the GPS icon, we need a clear indication of active microphone, speaker and other hardware modules while running in the backgorund.