I eventually figured this out.
If you click on the "volume" icon (a speaker icon, at the bottom-right of the desktop by default in Kubuntu), there are two tabs; "Devices" and "Applications." If you click on "Applications", and then click on the sandwich icon (three bars) that opens the settings, you'll find that "Built-in analog speakers" are the default setting; you must select your Bluetooth device manually.
Would be a nice fix in the next version of Kubuntu to have a plugged-in Bluetooth device be the default, as this is the behavior we see in nearly all OS's.