Diagnosis
The TV is likely receiving an audio format from your external device that its internal speakers can’t decode, like Dolby Digital or DTS. It’s an HDMI handshake issue.
How to Fix It
1) On your TV, go to Settings > Sound > Expert Settings > HDMI Input Audio Format. Change it from ‘Auto’ to ‘PCM’. This tells the TV to request a simpler, universal audio signal. 2) If that doesn’t work, go into the sound settings on your external device (PlayStation, cable box, etc.) and manually set its audio output to ‘PCM’ or ‘Linear PCM’, not ‘Bitstream’.
If That Didn't Work
Check your TV’s sound output setting. Press the Home button, go to Settings > Sound > Sound Output. Make sure it’s set to ‘TV Speaker’ and not accidentally switched to an external audio system like ‘Receiver’ or ‘Bluetooth’. Cycle through the options.
Heads Up
If you later add a soundbar via HDMI ARC, you’ll need to go back into the HDMI Input Audio Format setting and change it back to ‘Auto’ or ‘Bitstream’ for the soundbar to get the proper high-quality signal.