The only thing you can do is replace the muffler and look for other exhaust leaks. You should check the exhaust system from the exhaust manifold to the tail pipes.
Well as a muffler gets older it gets louder due to rust and loss of packing. The best solution is just to replace it with a new one. However if it is not too rusted sometime they can be rebuilt.
That usually means that it is loose and not doing its job as well as it should be. Waring they can and will fall off it that is the case and they can destroy other cars. You should go have it replaced.
