Sound issues Samsung series 6

Question

I have recently bought a samsung seriues six 37" tv and find that the soudn quality coming out of the TV speakers seems to intermittently lose quality. I am also unable to alter the sound settings to dolby settings as the menu option is not highlighted. In the menu it suggests somethign about connecting an optical cable but is that not only for an external audio system? I am currently viewing via my HD virgin cable box set at 1080