Q: Combining audio output to Bluetooth Speaker & Cinema Display

I would like to have the output of my Macbook Pro go to both a bluetooth speaker as well as the speakers in my cinema display (which is, obviously, connected to the MBP).

I have referenced this article: How to combine multiple audio interfaces by creating an aggregate device - Apple Support

However, I only get sound out of the Bluetooth speaker (having selected the new combined output device).

Clearly, I haven't configured something correctly...

Thanks!