Rechargeable C batteries, Solar? C Adapter Shells?
You could hook a solar charger up to C batteries. Charging is slow and the solar cells couldn’t power the speakers by themselves unless you invest in a lot of solar cells. They’d still quit when the sun went down.
I don’t know of a store that sells AA to C adapters but you might try a camping store. The problem here is that the AA batteries don’t have as much capacity and might not sustain the load.
A better idea might be to run a low voltage wire from another circuit in your house.