Sorry, I just realized I subtracted 514 from the analog data in code, so the real readings were 0 actually...
I think I also had the diode in the wrong way. The biggest issue however was the adapter I used. I basically went from a 3.5mm TRRS to a 6.3mm TRS to a 3.5mm TRS, to test it with an old phone before connecting it to my expensive audio equipment. The last adapter was probably broken, so there was no connection.
I think I might end up needing a higher resolution adc because the audio levels vary so widely.
I have an ESP32 module lying around which has a 12bit ADC. But it runs at 3.3v rather than 5v. Would that mean I have to change the resistors?
Also, just to keep it simple, could I get away with handling the peak detection or envelope in code on the ESP32 or is the processor not fast enough for that?
Edit: Tried adding a step down converter to turn the 5v signal into a 3.3v signal, but it doesn't seem to work. I mean I'm still connected to the 5V Arduino, but I would have expected to get a slightly lower reading on the analog pin. Instead I'm getting 0.
Edit2: Found something about voltage dividers on the Internet and tried to make one out of a 1.8k and 3.6k resistor and added that between the Arduino pin and the circuit. That seemed to have lowered to readings on the Arduino, so I tried it with an ESP8266 module (didn't wanna risk frying the ESP32 module yet). Well, it seems I have fried that ESP8266 module, I can't flash it anymore and there is no serial ouput. I tried it with another ESP8266 module then and the same thing happened. Although the second one still reports to the serial console for a few seconds before dying. I tried adding more power using my lab psu in case the USB power wasn't enough, but that didn't fix it either....