I'm away from a mac and too lazy to do this, but have you tried writing a little program to poll the audio configuration? Device name, number of channels, sample rate, etc.
It could be Chrome is just checking the device configuration for input/output at the same time (it needs the latter for the audio APIs) and the service might be pinging that as "microphone activity" when really they're just checking the available devices and their settings.
It could be Chrome is just checking the device configuration for input/output at the same time (it needs the latter for the audio APIs) and the service might be pinging that as "microphone activity" when really they're just checking the available devices and their settings.