IGNORE THIS - IT DOESN’T WORK (SEE NEXT POST)
It’s okay - I figured out what’s happened.
For anyone else, who prefers the traditional layout, but who also wants the sound indicator - the following is what has happened and what you need to do to get the above traditional layout with sound indicator enabled I have mentioned:
Firstly, the indicator is installed. But, it is not enabled on the traditional layout by default. Neither is is available to enable on the traditional layout via mate Tweak. However, it is enabled by default in the “contemporary” layout. But, the contemporary" layout has the contemporary menu plus the panel-based currently active application menu, like with Unity.
So, the way to “fix” this (if that is what you want) to have the traditional layout with sound indicator enabled is to initially load the "contemporary layout. Then, remove the contemporary menu and panel based applications menu bar from the top panel. Then, add the “custom menu” (which is, in fact, the traditional Mate menu) to the top panel and position it on the far left.
At which point you are done.
Takes a minute to implement, So, no big deal.