If it's really really small, clear fingernail polish works well. Shoe Goo works great for larger tears. I've also used RTV in a pinch, but beware - there are chemicals in the gasses released by curing silicone that actually weaken and eat away at the foam used in surrounds. That's also why it's wise if you use RTV to seal a box, let it dry for a day or 2 before loading subs. But i wouldn't really fear it for a small tear, the damage is already done.