1368 W/m^2 solar energy on earth on average, or 0.137 W/mm^2 = 137 mW/mm^2. Let's assume 15% solar cell efficiency, so you'll get 20 mW/mm^2 out.

500mW / 20 mW/mm^2 = 25 mm^2. That's how large cells it be exposed to the sun.

Cover both the headphones and cables, and you'll be able to charge any batteries enough to survive the night.