TV showing picture but no sound

Check the on screen sound set up menu to make sure the speakers are not set to OFF. If the setting is OK, then attach the powered PC speakers to the Audio Line out RCA jacks (Red and White) to see if you can get the sound there, if you do, then the problem can be in the power amp circuits, if you do not get the sound at RCA jacks, then the problem can be in the sound processor circuit which you will need new main board. Please post back what you find.
Parts: www.shopjimmy.com
http://www.fixya.com/support/r5093881-lcd_flat_panel_tv_troubleshooting_guide
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Could be the cable box is bad,u should check all other tvs in the house are find,if it have sound? Yes,tries unplug the tv and the cable box about an hour.Replug both back in.Let the cable box upgrade it software.The tv it now resetting back to it factory setting.Go into the tv Menu,go to AUDIO setting,go down to reset the AUDIO setting back to factory setting.The tv should have sound now.Not,than the Tuner board is dead,that the board take in the tv broscasting cable signal plug into it.Tries websites like Shopjimmy.com,Ebay.com to buy a refurbish Tuner board for the replacement.