DIAGNOSIS The TV's audio output got switched to an external option, or a software glitch muted the audio processing.
FIX First, check the audio output. Press the Home button, go to Settings > Sound > Sound Output. Ensure it's set to "TV Speaker," not "Receiver (HDMI ARC)" or "Bluetooth." If it's already on TV Speaker, change it to something else and back again.
IF THAT DIDN'T WORK Perform a sound reset. Go to Settings > Sound > Expert Settings > Reset Sound. If that fails, do a full power cycle: unplug the TV, hold the physical power button for 30 seconds, then plug it back in.
HEADS UP If you've ever used an HDMI ARC port (HDMI 3), the TV can get stuck trying to send audio to a non-existent soundbar. Disconnecting any device from HDMI 3 can sometimes resolve this immediately.