Philips Matchline TV - lost sound?

my philips matchline tv has suddenly lost sound and there is an icon top left showing a white loudspeaker with a red cross through it. This is not the usual mute icon which appears as well if you press the mute button on the remote though this has no effect on the sound. It happened while changing channels using a Virgin plus remote and I wonder if some internal mute function has accidentally been set but I have no idea how or how to reverse this.

I have checked all connection cables and have looked at the TV menu but there doesn't seem to be an option for internal speakers.
Any advice gratefully received.

KH
May 2011