prefs = {

"profile.default_content_setting_values.media_stream_mic": 1,
"media.default_audio_capture_device": "Device ID" 
}

Device ID can be found on Chrome setting using Dev tool. In my case, the microphone A is

{0.0.1.00000000}.{6c057a49-4423-4c97-8806-f51e62014e85} Write the code like this:

 prefs = {

"profile.default_content_setting_values.media_stream_mic": 1,
"media.default_audio_capture_device": "{0.0.1.00000000}.{6c057a49-4423-4c97-8806-f51e62014e85}" 
}