Arduino ADC converter

Hi guys! I'm trying to convert an old analog synth into a digital one. the synth is supplied with electrical current and has a speaker. I'd like to remove the speaker and make the Arduino read the values that should go to the speaker.
What do i need? an external chip, an adc converter or whatever?

Can I only connect to the ground the black wire that should go to the speaker and red one through a capacitor and then to the arduino? then can i read the values from the serial monitor?

I upload a photo of the synth

Reading the output of an analog device using a digital device does NOT make a digital synthesizer.

Besides, unless you limit your synthesizer to low frequencies only, I doubt that you will be able to get most Arduinos to sample fast enough.

Some people can do this but it is not trivial.

so any advice to read the values of the speaker?

Use a microphone module?

Use a microphone module?

which way?