Superb speakers not working

Hi,

I have a 52 plate superb with the built in satnav/stereo unit with cd changer. A few weeks ago I noticed the sound had stopped working on the front pasanger side, now I have no sound at all in the front as both sides have stopped working. Today I thought I would take a look and see if the speakers had blown etc, started working out how to take the door panel off, then noticed the doors had tweeters as well which are not working either. Its unlikely the tweeters went wrong at the exact same time as the main speaker did at two different times, so im thinking something else must be at fault.

Does anyone have any idea what this could be?

cheers

Tony