If it really is a small tear, very carefully push the edges back into place. Then brush both the front and the back surface with some clear nail polish. Yeah, thats what I said, nail polish. Don't use tape. It won't stay on, and it may make the tear worse.

If the tear is too big for this fix to work, you will have to get the speaker re-coned. A pro shop should be able to handle this for you, or at least get you the address of the manufacturer, so you can send it in yourself. It shouldn't cost a lot; its a fairly common procedure.