Arduino to Home Speakers I know little about electronics in general. I wanted to connect Arduino, and play a tone using tone() through my home speakers.

I've used a 1/8'' audio jack and connect ground to ground and left/right to pin 2. Run it and it was too loud.

Then I read the specification and it says,

Input impedance = 20 kOhm 

Input sensibility = 400mV +/- 50mV

Arduino sends 5V so that should be a lot. Did I damage my speaker? They still work.
It's probably not damaged, but you shouldn't do it again.

To get the 5V signal down to 400mV you need to create a voltage divider. As such you will need a:

5V / (400mV / 20kohm) − 20kohm = 230kohm

resistor between the pin and the input. 220kohm should be close enough.