So, while tightening up some of my audio notifications, I noticed that they weren't playing on L/R paired Sonos speakers. Going into my rules, I had selected only 1 side of the L/R pair. Once I selected both under either Speech Devices or Music Devices, I realized I had to have both selected for notifications to play.
So, pro tip (if not mentioned before): When you are trying to push notifications to paired L/R Sonos speakers, select both speakers and notifications should work properly.
