Pins 2 and 4 on connector CN2 are the audio inputs. FL and FR are probably intended to be the left and right audio inputs, but they are simply connected together, so you only need to use one.
If you do want to connect both the left and right inputs, you should add a series resistor to avoid causing issues with the rest of your audio system. Using a 1kΩ resistor on each input should be enough.