With a 9V single supply and single-ended analog amp I'd expect an 8 ohm speaker to draw up to 1.25W
maximum with an ideal amplifier and sinusoidal signal, and upto 2.5W with a square wave.
Certainly not 9W... An AC signal constrained to a peak-to-peak range of 9V is not a 9V signal
As you have a 15W rated speaker I'd imagine you want to get the most out of it.
For this you'll need to use a bridge-mode amplifier, so you can get upto 18V peak-to-peak
and a sinusoidal power of upto 5W and square wave upto 10W
For best efficiency a class-D amplifer module capable of handling 9V supply would be a good
You then have to condition the signal into the class D amp to be basically analog and of the
You could take a square wave signal directly and feed it through a suitable MOSFET motor driver
H-bridge to drive the speaker - this is effectively digital input class-D.
Your decision to use a free-wheel diode is understandable - however speaker coils
are much more resistive than inductive and it is not generally needed for analog audio, and
is unlikely to be needed for digital switching of a speaker either.