The TV's menu is terrible and the audio settings are very minimal. I don't think there's an option to turn the optical output on or off. The only audio settings I've found in the TV's audio menu is to choose between PCM or Dolby Digital and then another part allows to choose Variable or Fixed...
I have a 5.1 home theater in a box connected to a TV via RCA (red & white) and strictly use the HTIB for audio. I have other components connected via HDMI to TV (DirecTV, Roku, PS3) and I get sound through the 5.1 speakers no problem. I just bought an optical cable to replace the old analog RCA...