No one has really answered the question with a proper fix. The problem is caused by Google Now. Go to google settings app>search&now>voice>ok google detection>turn off. Sound will then work. You don't need to clear any radio cache etc. You can also get to google settings in pretty much any google app. Just disable voice search option. You can double check by turning it on again while the radio is running and it will mute the sound again.
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