how do I get the volume back?

Hi
I have a Proline 15" TV/DVD combi model No TVD 1550F.
When new I set he channels without any problems but lately the channels have lost thier sound. The picture is perfect but no sound. IF I re set the channels on thier current setting I.E. re set BBC 1 on Channel 1 the sound is still missing but if I go further up the channel numbers I.E. if I set BBC 1 on Channel 21 then the sound is OK.I believe the problems started when I used a video player that was in the same room as the tv and may have been on the same remote frequency therefore when pressing the video remote it has activated the TV. Is this my problem and how can I re set the TV.