I have been using Macs of every flavor forever and I am embarrassed to say I did not know it is possible to combine audio outputs. In retrospect it is an obvious feature but somehow I never came across it.
My Mac has multiple speakers - one each in several monitors, built-in speaker, and an external speaker system through the audio output. At various times I have experimented with which works best depending on how I arrange my office.
It turns out that there is a macOS supplied utility called Audio MIDI Setup which lets you create a virtual Aggregate audio device which combines any or all physical audio output devices.
Am I last to the party or is this an oversight for others too?