Diagnosis
This is usually caused by the CEC control feature (called 'VIZIO TV' in the settings) getting confused by signals from your other devices, like a game console or streaming stick.
How to Fix It
You'll want to turn off the auto-input switching. Grab your soundbar remote. Press the 'Menu' button, then use the arrow keys to navigate to 'System' and select it. Find the option for 'VIZIO TV' or 'CEC' and change it to 'Off'. This stops the soundbar from trying to automatically guess what you want to listen to.
If That Didn't Work
The issue might be coming from your TV. Go into your TV's external device manager settings (often called HDMI-CEC or Simplink or Anynet+) and turn that feature OFF for the port the soundbar is connected to. This breaks the automated handshake chain.
Heads Up
Turning off CEC means you'll lose the ability to control the soundbar's volume and power with your TV remote. You'll have to use the soundbar remote separately, which is a trade-off for stability.