adc - How to give analog input from 3.5 mm jack to FPGA (Spartan 3e)

I am working on a voice transmission project by using Spartan 3e. My code works just fine. My problem is that I give analogue input to adc side by using a potentiometer and see the changes (as digital) on leds ( digital output is assigned to leds). Here is the problem,I cut a headphone into two and connected 3.5mm jack side to my phone to play music. There are 3 cables which are golden, blue and red. Golden one is connected to "GND" pin , Red one goes to "Va" pin (at adc input). I didnt touch the blue one.

When I play music,I see no change on leds despite the fact that I did see by using potentiometer. What am I missing ? Do I have to setup an external circuit before giving the analog signal to Va input ?

Thanks in advance.