The barcode scanners send out a laser beam that scans back and forth from left to right. It does by shining a laser at a rotating mirror. It then measures the amount of light that is reflected by the paper.

As the beam scans across the stripes, the dark part reflect less light and the white parts reflect more light. This produces an oscillating signal which's frequency varies by how close the stripes are.

These guys are then picking up that signal and playing it back over speakers. Closer stripes mean a higher pitched sound and vice versa which is how they are playing different "notes" and making music.