Chabrolly,
I had this problem with my P38.
The radio does not have a manual "MUTE" function. It only operates when it gets a signal from the phone kit when a call is in progress.
However, if the steering wheel button contacts become dirty, they send a dodgy signal to the radio, which puts it into "MUTE" mode.
You just have to keep pressing the buttons to clear the fault.
As The Mad Hat Man says, you can replace them, but I just shouted at them, gave them a damned good blow out and started using them regularly and the problem went away.
Good luck!