Where I can find acoustically clear foam?

You are looking for a product called "reticulated foam". That is foam with all the cells "popped" (no bubbles) so water and sound would just run through it. Wilson and many other manufacturers use it for speaker grills. It will stick quite nicely to Velcro.

You can buy it in various densities and sheet thicknesses at most foam suppliers. Generally, the foam color indicates its density, so you don't have much choice there.

I'm not sure if it can be sewen (to make a sock) but there are special adhesives that you could use to join it at the seams.
.

Guilford of Maine - http://www.acousticalsolutions.com/products/fabrics/guilford.asp

I don't know how big these speakers are but the foam filters made for air conditioners that are about 1/4 inch thick is the type I've seen used on other speakers. I think the type is called open cell foam

Hope this helps a little

Great idea!

Thank you.