Splitting iTunes/Music app and System output: OG HomePods vs Montor

MacPro 6,1 (cMP) - BigSur 11.7.6

Stereo pair OG HomePods

For several years, I've had my system configured to output the iTunes/Music app though my HomePod stereo pair....and output all system sounds through my monitor speakers. But, if I'm honest, it's worked so well and gone so long without any difficulties that I've forgotten how I did that.

Recently, those settings seems to have been forgotten. Now....I can play everything through one or the other....but not split as it had been.

How do I get my split output back?