It's quite easy to use one of these solar lamps to drive a low current load like an AVR tone generator. This exact setup will only work with the very cheapest solar garden lamps, anything bigger and more expensive might provide too high of a voltage at these low loads.
Here is a link to a zip file with the C code and makefile for the project, as well as a spreadsheet i used to set up the delay timings for the different notes. Since i only know how to code in crappy C, the code is in crappy C.
http://ghlargh.com/solar_music.zip
