This may be due to the incorrect sound system setting in the TV menu.

Follow these steps to verify if the sound system is set correctly:

Select the channel on which the audio is not available.
Press the 'MENU' button on the remote control to access the main menu.
Press the cursor down to select INSTALL and press the cursor right.
If the Parental lock is activated, the screen will prompt you to enter your access code. If you have forgotten your access code, refer to the FAQ (I have forgotten my 4-digit access code. How can I reset it?).
Press the cursor down to select MANUAL STORE and press the cursor right.
Press the cursor down to select SOUND SYSTEM and press the cursor right.
Check if the sound system is set to the correct sound system.
If you are not sure of the sound system used in your location, press the cursor up/down to select AUTO and press the 'OK' button for confirmation.
Press the cursor left to return to the manual store menu.
Press the cursor down to select STORE and press the cursor right to YES.
Press the 'OK' to store the settings.
Press the 'MENU' button to exit the menu.

If the sound system is set to AUTO and audio is still not available:

When no improvement is observed then it may be due to a bad incoming cable or antenna signal.

Contact your cable provider or broadcaster for information on the broadcasting quality of the particular channels