HDMI Audio

I watch TV on my All-In-One by switching to HDMI mode, with the HDMI IN cable connected to my PVR. When I start watching TV via the PVR to HDMI IN port, the computer  internal speakers do NOT sound , as I have external speakers always plugged into the audio OUT jack, which cuts off the internal speakers.  I use headphones plugged into the PVR audio OUT to access the TV audio, allowing me to watch TV & not disturb others in the room. 

Part way through a TV show the internal speakers suddenly start to sound, anywhere from 15 minutes to an hour into the TV show. I do not understand why the internal speakers behave this way. Is it a property of the HDMI IN port to route the TV audio to the internal speakers eventually, even when external speakers are plugged in ? 

My system is: Inspiron 27 7710 All-in-One running Windows 11 HOME.