headphone input to arduino for FFT usage
The mic amp gives out 2Vpp max. Audio outputs will vary between devices, but this does not matter as the devices also have user controllable volume control. The schematics given in the Arduino forum link are OK to start. The AVR can work with 3Vpp signal when powered from 3.3V. The 100k resistors will bias the ADC input to 1.65V or half-supply voltage. With that high resistances, even 100nF cap will provide audio down to 31Hz or so, and it will work with 10uF as well. Only thing you might want to consider is a low pass filter to limit the audio bandwidth to about half of the sampling rate to avoid aliasing.