BBC entertainment, Knowledge, World & SKY has the same problems on mine every now and then. I have found 2 ways to remedy this.
1. While one the affected channel turn the decoder OFF on the remote, Turn it back on after maybe 5 seconds and it all sorted but only until you change the channel and return to the BBC channel then its back to NO SOUND
2. On your remote press MENU, the press 3 then 3 again and change from Manual to Automatic. Press 4 to accept settings and you will once again have sound on the affected channel but only until you change the channel and then return to the channel and we are back to square 1