DIAGNOSIS That's often the TV's "Auto Volume" or "Sound Optimization" features kicking in. They're trying to balance loud commercials with quiet dialogue, but they can get too aggressive and cause dropouts.
FIX Go to Settings > Sound > Expert Settings. First, turn off "Auto Volume." Next, turn off "Sound Optimization" (sometimes labeled "Adaptive Sound"). Set "Sound Mode" to something standard like "Standard" or "Amplify." This gives you a raw, unprocessed audio signal that should be consistent.
IF THAT DIDN'T WORK Check if any of the special audio formats are causing a conflict. In the same Expert Settings menu, set "Digital Output Audio Format" to PCM. Also, make sure "HDMI-eARC" is turned off unless you're actually using it with a soundbar.
HEADS UP The built-in speakers on thin TVs like this aren't powerful. If you're still having issues after turning off the processing, it's worth looking at a basic soundbar. The TV's optical audio output or HDMI eARC port will give you much better, more reliable sound.