Diagnosis
This is almost always a handshake issue between your TV and the soundbar over the HDMI connection. The two devices are briefly losing sync.
How to Fix It
Start with the digital equivalent of turning it off and on again. 1) Turn off your TV and soundbar. 2) Unplug the HDMI cable from both the TV's ARC port and the soundbar. 3) Unplug both devices from power for 2 minutes. 4) Plug everything back in: power first, then reconnect the HDMI cable firmly. Power on the TV first, wait for it to boot, then power on the soundbar. This re-establishes the connection from scratch.
If That Didn't Work
Try a different, high-speed HDMI cable. The one you're using might be borderline faulty or not have the bandwidth needed for a stable ARC connection, even if it works for video. Look for a cable labeled “High Speed” or “Premium High Speed.”
Heads Up
Some TVs have a specific setting in their menu to enable or control the HDMI-CEC/ARC function. Make sure it's turned on. It might be called “HDMI Control,” “SIMPLINK” (LG), “BRAVIA Sync” (Sony), or “Anynet+” (Samsung).