If it's anything like my large screen it 'smack's' of a setup/remote issue. I'd check the audio setup in the MENU options and see if there's an internal/external audio option or possibly audio by coaxial option selected. When you've tried everything else you could try resetting the default for your set but you'd lose all your presets and programmed stations. If the default is not in the menu or the manual you may have to call the manufacturer for the reset function. Also..on my set weird things happen when I have the closed caption options selected. BTW Don't feel you have to rate my answer unless it helps. Hope it does.
You may have the lack of a physical contact point...the RCA type male plug may not be fitted into the transmission connector for external speakers. If internal speakers are present the setting/selection could be at 'MUTE', 'EXTERNAL' WHEN THERE ARE NONE INPLACE/CONNECTED OR POSSIBLE SET TO 'SAP' WHICH IS NOT BEING TRANSMITTED FOR THAT CHANNEL
I don't know if this will work but it may be worth a try, in the menu's audio settings there is a place where you can turn on or off the audio out to either the TV or external amp. The problem might be there.
I have a 1985 to 1990 60 inch projection tv that will not turn on, if you push the switch the button that let you know that is on stays on for few seconds then it turns back off.