How can I fix the sound on my tv?

My Sharp LCD Flatscreen tv has developed a fault with the sound. It has been going on for about six months now, but has gradually got worse. What happens is you hear a slight crackle then the sound goes off completely but the picture is still fine. It used to correct itself by turning the tv off and on with the remote (usually) or sometimes at the socket which normally turned the sound back on. However, now it is only playing sound for about twenty seconds before it goes off and when you switch it on and off it's fifty fifty if the sound comes back on or not. Is this problem fixable or would the best bet be to buy a new tv? The tv set cost about £850 three years ago but I know you can buy good tvs for about £400 these days.