zallesov:
... what is amplitude of a signal from an average cell-phone player? ...
I don't know of any standards. Here's a guy testing the audio output of an iPhone 5: http://www.kenrockwell.com/apple/iphone-5/audio-quality.htm. According to him, full output is about 1.4V peak-to-ground, with an output impedance of 4.5 ohms. With a 2.5V offset, that'll give ADC read ranging between roughly 225 and 800, at full output.