concert cd player

I have no operating manual for the cd/radio. Sometimes when I switch it on all speakers work. Sometimes when I switch it on , only the rear speakers work.
Is there a setting (not the fader) or do I have a unit needing repair?
Alan (1999 A6 1.8T)
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I have the same problem!
Mario, 2002 A4 1.8Tq

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