You’re taking the wrong approach to this. If they’re consuming content in public via the speaker, they’re including you on the consumption. Join in the conversation if they’re on a call. Comment on the music. Sidle on over next to them and ask them to share the screen if it’s a video.

“Oh, you weren’t including the rest of us? Don’t use the speaker then.” Look into their eyes, and hold that stare for much longer than is comfortable.