Zener diode has no voltage because
you should put there what you need.
As for the other components you
should put again what is needed.
I only ment in general, disregard
the component markings and ratings.
A speaker is physically overloaded which is affected by frequency and its enclosure, and is electrically overloaded by heat.
The physical overload occurs at low frequencies where the voice coil hits the magnet structure.
The electrical overload is affected by duration since a "30W" speaker might melt if its continuous power is 40W but it can easily handle peaks of 100W and more.
Your simple circuit ignors frequency and duration.