Why isn’t there any stereo sound for the FM radio on devices running Android 6.0 or higher?

You may experience that the FM radio sound of your Xperia™ device has changed. This is because the FM radio has been changed with Android™ 6.0 (Marshmallow). The result of the change is that the FM radio is now allowed to automatically select between mono and stereo audio output, depending on the signal-to-noise (SNR) of the incoming signal.