I'm working on a simple audio circuit which takes two different signals from mp3 players (earphones output), mix them up and outputs to an amplifier/speaker. But before mixing the signals, I need to measure the onset of Signal 1 with a microcontroller, in the case an Atmega328. I have little experience with audio and amplifiers so I don't know how to split Signal 1 from Signal 2 in order to read Signal 1 only.
I have also added a simple voltage divider for DC offset so the Atmega328 won't read negative voltages. Below a simple schematic of what I have so far: