Perhaps you get notification sooner if you turn off support for images. But keep color note either in guidelines or techniques. Adopt proposal for 7.13. Screen readers used to look for borders. They may not anymore.
Worst case is an image map.
Look at techniques document. But you lose the rest of the presentation. Allowing user control of presentation and layout. But there are applications that are delivered on the Web that have volume controls on them. In Windows, you can have a lot of different sources of sound.