I d like to set up a very unobtrusive solar, motion-activated mp3 player at my parents grave. Mp3 could be buried. What do I need?

What you will need first is permission from the cemetery.

Provided you can get that, you will next need an absolutely watertight,

tamper-resistant enclosure for the MP3 player.

Such an enclosure Will Not be "unobtrusive" unless you can bury it.

How to have a speaker that can be both weather and tamper resistant, I don't know.

And the solar panel would have to be rather large,

probably on the order of at least 18 inches square or the equivalent area in another shape.

Really, the whole idea seems doomed from the beginning.....