Diagnosis
The TV isn't decoding the audio signal coming from your external device over HDMI. This often happens because the external box is set to output an audio format the TV speakers can't handle, like Dolby Digital.
How to Fix It
Start with the TV remote. Press the 'Settings' button, go to 'Sound', then 'Advanced Sound Settings'. Look for 'Digital Audio Out' or 'SPDIF' settings. Change the 'Digital Audio Format' from 'Auto' or 'Dolby Digital' to 'PCM'. This sends a simpler, universally compatible stereo signal. If that brings sound back, the issue is with your source device.
If That Didn't Work
The problem might be the specific HDMI port. Unplug your cable box and plug it into a different HDMI port on the TV (like HDMI 2 instead of HDMI 1). Also, try a different HDMI cable if you have one handy—a faulty cable can carry video but not audio.
Heads Up
Some gaming consoles and Blu-ray players send high-quality audio by default. You might need to dig into that device's audio settings and manually set its HDMI audio output to 'PCM Stereo' or a similar setting for it to work with the TV's internal speakers.