You'd be surprised how modulated the sun is. Try listening to that, probably be your best source for light to listen to. Don't worry about DC bias, it's not as hard to deal with as you might think. Just use a few lenses from true 'grey' sunglasses, or something that is equally 'grey' in the desired passband. The filters you put over the element itself will be the true determining factor of what you get as the output, the straight DC bias from the sun alone is enough to completely swamp nearly any kind of photo detector that's naked to the light, filters are where it's at.. Also record the sound even if you don't hear anything, and then run it through audio software to speed it up. Most light modulated sources aren't audible at normal rates, but create eary and almost whale like tones when sped up. I've listend to some Sun audio clips, they definitely get your attention.