The speaker takes an RCA Input at the bottom (which is the AMP), this is then carried up through the speaker to the crossover and then onto the speakers, unfortunately it will be varying volume from the AVR.
OK, I see. Those are active speakers. I'd try using that line-level (RCA) signal because you won't have to worry about over-voltaging the Arduino. You should get around 1V with the music "blasting" but you'll just have to experiment to see if you get a usable signal at normal listening levels.
It would also be worthwhile (as an experiment) to connect to the line level signal with the speakers disconnected and the volume cranked-up.
You can also do a basic analogRead() to see what kind of raw readings you're getting. Just remember that you're reading a continuously-changing "wave" with positive & negative* peak once per-cycle and two zero-crossings per-cycle, so the readings will "look random" but you should be able to pick-out the peaks manually/visually or in software.
And/or depending on how it's going for you, you might want to experiment with a little [u]microphone board[/u] so you can better-duplicate the project you are "copying" before customizing it the way you want.
You'll probably end-up using a voltage divider at the amplifier output and it would be "smart" to add the protection diodes. The resistor values should be much higher than the 4 or 8 Ohm speakers. Resistor values that add-up to around 10K Ohms are a good starting point. But the values are not really critical... It's the resistance ratio that makes the voltage divider work.
The protection circuit alone (without the voltage divider) is actually OK to experiment with, but if the signal is too hot you'll clip (hard limit) the readings which will create distortion in the FFT and give an "invalid" spectrum result.
On the output from the AMP i am getting just 4 volts, i currently do not have any voltage divider in place, i am completely new to this but i am confident i can implement this fairly easily.
I assume that's DC voltage? That's a little strange. I assume you're measuring that relative to the Amplifier's ground? If you're measuring that without the speakers connected it could be "leakage" and it might go-away when you connect the speakers.
There shouldn't be any DC voltage across the speakers/drivers. That would be a defect.
Any AC voltage depends on the signal level so you'd only read a constant 4 VAC with a constant test-signal and of course you'd hear it (if it's in the audio frequency range).
You mentioned that the Arduino is always reading relative to its ground, how the display & ESP32 are powered, is that i am taking 10vAC from the Old Display wiring, and have a simple LM7805 to convert it into 5vDC to power the equipment, is this an undesirable way to do things in terms of the ESP reading relative to its ground?
Well... A 7805 doesn't work from AC.
If the AC is rectified & filtered that's OK. And of course, everything needs a common ground. And since you are getting power from the amplifier, everything already has a common ground.
- Do you understand about biasing the input so you can read the negative voltage? The chip can't read negative voltages, in fact I can be damaged by negative voltages, and FFT needs to "see" the whole undistorted waveform.