I also have this pop sound when video is played in the Browser. Didn't happen on Stock Android as much as I remember.
It doesn't happen when Video is played in Apps.
Even when sound for Websites is muted, the pop appears as long as a video plays in the browser.
I think somehow the browsers audio is dominant over all other sound sources, so it interferes with the audio when video or video ads are played. For example if you have a music player running and at the same time use the browser, the pop sound appear.
It didn't help using different browsers like Brave or Google.
My phone is a Pixel 9.