Assess the damage. Depending on the severity of the cut, you may need some other scaffolding material (aside from RTV silicone) to enclose the hole.

This is especially true if a chunk of the rubber material is completely missing from the rubber surround.

In this case, all we need to do is cover up the tear with RTV silicone in order to make the subwoofer almost as good as new — functionally, at least.