Wow! Those rotted rings make me wonder what kind of solvents were floating around the atmosphere where those particular iMacs lived. I'm imagining lots of chlorine, maybe hydrocarbons?

AFAIK those rings don't have any critical audio characteristics, they just have to support the diaphragm through the full distance of it's potential stroke, maintain the seal to the rear chamber, and return to neutral. However, they could be pretty hard to get right if trying to build one from a mould.

Very similar speaker units have become quite common in portable iPod extension speakers, often surprisingly cheap. Sadly, replacement might turn out more feasible than repair, at least where the rings are significantly rotted.

Rick