What could be the problem?

The radio is on (factory) , I can change stations and so on but no sound from any speakers.

Please help

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2008 ML320 CDI
2007 GL320 CDI
2005 Audi A6 (sold)
2001 E320 (sold)
2004 lexus RX330 (sold)
2004 Honda CRV (sold)
93 190E 2.6 (sold)
97 E320 (sold)
2000 Honda CRV (sold)
91 350SD (sold)
90 E300 with amg body kit (sold)
83 300sd aqua marine (sold)
81 300sd dark blue (sold)
81 300sd gold (sold)
81 300sd brown (sold)