This site: http://www.retrevo.com/support/Maxent-MX-42VM10-TVs-manual/id/278ag123/t/2/ has the ops manual for your TV. If you don't have one, try that. Check if there is a way, perhaps a setup for surround sound, that disables the internal speakers. It would be somewhere in the TV's menus.
Answers & Comments
This site:
http://www.retrevo.com/support/Maxent-MX-42VM10-TVs-manual/id/278ag123/t/2/
has the ops manual for your TV. If you don't have one, try that. Check if there is a way, perhaps a setup for surround sound, that disables the internal speakers. It would be somewhere in the TV's menus.