I think silicone will work if its a rubber surround, dont know how it would work on foam tho. One of my friends did it to some old orions and it worked fine. What kind of sub is it?
Catch it early b4 it runs on ya if it is a rubber surround. Eventually you'll probably want it replaced. Not hard to do yourself. Save some $$$ also, especially if it's a high end sub.
Those have rubber surround if im correct, right? Put a little dab of silicone from the inside of the sub and rub it around make sure you cover it completely so no air can force through. But be careful not to put too much, just keep the amount you put on there about the same thickness of the rubber surround itself and let it dry overnight. After all of that just kinda push on it a little and play with it to make sure its gonna stay sealed.