Channel 1 is a standard audio output from a smartphone or PC. I have read that LED D1 should flicker depending on a music. But is it even possible? I don't think so. It is always on because there is a constant electrical connection between LEDs' pins and power supply.
Now more difficult part of this circuit. I guess that Q1 is used to amplify the voltage of audio input in order to power optoisolators' infrared diode. Am I right? I have search through common transistor configurations and never seen even one configuration like this. I would like to know how does it work.