Is it a Clarion radio?
I have a similar problem. The ext mute seems to me to be a problem with the remote control stalk?
My radio doesn't show this message, but it plays fine for 10 mins to an hour, then loses the signal. I can hear it (just) but it's very faint and hisses and 'pops' as if searching for a radio station. I tried to change stations, but it just stays on the original station. Turn it off for 10 mins, turn it back on, and it's fine ...... for a few minutes, then 'goes' again.
Clarion quoted me £90 to fix it. LOL!!!!!!
They're avvin a larf!
Dexy Owl, February 2010