You don't happen to have green glue to put between the box and the extra layer? It would help a little better than the silicone , but for the back of a set of speaker boxes, I don't know if there is THAT much benefit to worry about it if you don't have GreenGlue.

Great call on the GE Silicone II in general by the way. The clear is a really good product to use for sealing up any gaps in a theater.

Farewell - June 4, 2020