Check your spark arrestor (sometimes easier said than done because the retaining screw nearly always has to be drilled out) first). There isn't much else inside the stock muffler that can become dislodged or broken. THe "baffling" consists of a welded inner sleeve with a nosecone and unless it rots through it's pretty much permanent.
On older bikes or bikes that have run too lean the outer screen tends to rot or burn off the spark arrestor (loud), the holes become enlarged (louder) or the inner portion drops or rots off alltogether (I can't hear you!).