When we first got our Miramar we were told it did not have a microphone. We usually drove with the rear camera on to watch the toad. No sound until one morning we got bird song and road noise from the speakers. This happened occasionally and eventually I contacted Thor and got the following information.
Go to the menu and touch to Rear Auxiliary icon. You get a screen that tells you not to use the screen while driving and if you turn the volume control you also get a volume bar at the bottom which sets volume for the rear microphone.
Works every time for us.
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