[chernofool]

You could probably get 2W from a wide over-ear band in direct sunlight. Maybe that's 2-4 hours per day, with indirect light scattered around the day.

If you get say, 2W*6h=12Wh/day, you could afford about 15mA/hr @3.3V. That might be able to power a set of wireless headphones with a reasonably-sized battery, if you left them on a windowsill in direct sunlight when you weren't using them.

Modern designs will probably use monocrystalline cells, but old solar-powered calculators used amorphous cells. Those are less efficient, but they can recover a trickle of energy from weaker light sources such as indoors or cloudy days.

I also wonder what sort of materials they'll use. Plastics don't usually handle excessive UV exposure well.