The audio system on my 2004 Mercedes clk 320 is muted. How can I reverse this,so that I can hear radio,cd,nav?

There could be several different issues occuring. As with everything electrical, you could try unplugging the battery's negative post, let it sit for 5 minutes, and reconnect it and see if it reboots. This guy on youtube gives and in depth way to trouble shoot the multiple systems which affect audio & could interrupt the signal. It's kinda like old Christmas lights. One 'bulb' goes out, it affects the rest down the chain...