Yeah, those foam surrounds tend to rot sooner or later depending on the environment they live in. There are places that can put new suspensions on speakers if the speaker is worth it. And there are places you can buy the foam rings (some of the replacements might even be guaranteed to last a long time) and try to do it yourself. Just google on- loudspeaker repair - or - loudspeaker repair parts - to find some sites for each option.
. Just be aware that the repaired speaker might not sound quite the same after the repair as with the original surrounds (but it might take a trained ear or even instruments to tell the diff). And you may as well have all the drivers that have those foam surrounds repaired at the same time as the others can't be far behind. If you looked close, you would probably find they are already hanging by a thread.
.bh.