Sound keeps on dropping on 32inch Bush flat screen?

We have a Bush 32inch flat screen tv which we've had for a few years now. We've had absolutely no problems with it until the last week. The sound keeps on disappearing and then returning. You can watch on channel with sound, but if you flick to another channel or watch something that you've taped, then there is no sound.

It also comes and goes during the same programme, so basically we cannot watch tv at the moment.

Do you know if we can just replace the sound card, or if we need to replace the tv?

Many thanks

Dy Nissen