DIAGNOSIS This is a common HDMI-CEC/ARC handshake failure. The TV and soundbar get confused about which device should be the audio controller when you change sources.
FIX First, ensure your soundbar is connected to the TV's HDMI 2 port, which is the dedicated eARC port. Then, on the TV, go to All Settings > Sound > Advanced Settings > Digital Sound Out and set it to 'Pass Through'. Next, go to 'Sound Out' and select 'HDMI ARC' even if it's already selected—this resets the handshake. Finally, turn off 'SIMPLINK (HDMI-CEC)' and turn it back on.
IF THAT DIDN'T WORK The HDMI cable is the culprit. You must use a certified Ultra High Speed HDMI cable for eARC to work reliably. Swap the cable between the TV and soundbar for a short, premium one. Also, on your external device (like a game console), set its audio output to Bitstream (Dolby Digital/DTS) instead of PCM, as some soundbars have issues with multichannel PCM via ARC.
HEADS UP If you use an external streaming device (like an Apple TV) and a soundbar, set the Apple TV to output audio in Dolby Digital format, not Dolby Atmos. The TV can pass through Atmos via eARC, but the chain is more fragile and prone to these handshake dropouts.