Ambient light on hand-painted interior


I’m having issues with lighting an interior hand-painted scene. I’d like the lighting to be realistic with ambient occlusion and soft shadows. However, the scene has no natural light source aside from the windows, which are hand painted and not transparent at all. What is the best way to achieve a decent light setup in this scenario? A cubemap does not do anything because the scene is all interior and closed off. Any tutorials or hints would be very appreciated! Thank you!