In article 1nhj6rw.woj7e71rtk443N%real-not-anti-spam-address@apple-
juice.co.uk, says...
I have an old pair of speakers whose foam grilles turned to dust.

Can they be replaced with any old thing? Or must I obtain special
tempered virgin linear response low distortion acoustic audiophile
expensive foam?

Open cell acoustically transparent foam will do it.
Something like this https://goo.gl/zkk5ir

And how difficult is this stuff to cut?
Difficult? It's foam and mostly made of air, you may have to learn how
to operate a pair of scissors or a craft knife. any small child should
be able to show you how to do that. A bit of practice on a bathroom
sponge and you should be good to go.

Save yourself the hassle, cut one leg off a pair of tights and pull that
over the speaker, could add to the aesthetics, fishnet speakers anyone?