Prevent built-in speakers from being used when macbook pro is closed

I have a 2018 MacBook Pro. I would like it to not play audio through the built-in speakers when the laptop is closed, but I can't find a setting for this.

Currently, I have the laptop plugged into an external monitor and power source, and no external speaker or headphones. When audio plays it currently comes from the built-in speakers even though the laptop is closed.

I usually just mute it, but on multiple occasions, I have logged in and some youtube video or netflix will start auto-playing and the audio will come through the built-in speakers.

How can I fix this?

I'd be willing to completely disable the built-in speakers if that's an option. I have never used the built-in speakers intentionally. I always either use headphones or a wireless audio system.