It kind-of depends on how & where it's leaking but caulk (or silicon seal) or glue should work. The pressures are low so it's not like sealing a tire or air tank where there are several PSI. Of course, appearance may also be important.
...If you can't hear it leaking while sound is playing, it's no big deal. Sealed speakers don't have to be perfectly sealed because only a tiny amount of air is going to leak-out or leak-in during the short-duration of a half-cycle, even at 20Hz.