If the foam has not deteriorated to the point of falling apart, meaning it feels week but no holes in them, you can revive them for a couple of years with clear latex. Take some normal latex paint that has been sitting around, Do Not Shake it, Open it carefully and you will find the latex has seperated from the pigment. Carefully paint on Latex without color smoothly and carefully with a small brush. Let the first coat dry before applying any more. But you will find the foam soaks up the latex and does quite a good job reviving it. I came from the old SpeakerCraft/Marcof Electronics out of Webster Groves Missouri. I have done this 100 plus times, it works well.
Good Luck, Tim