French site Nowhereelse.frshares [Google translation] a pair of photos of what the original source claims are speaker enclosures for both the upcoming iPhone 5S and a future iPhone 6. The two parts are similar in appearance both to each other and to the corresponding iPhone 5 part, but do contain several differences.
Photo of claimed iPhone 5S and iPhone 6 speaker enclosures with iPhone 5 part superimposed
It is of course impossible to tell for certain whether the parts are indeed legitimate and whether they are intended for the upcoming iPhone models as claimed. Nowehereelse.fr has published a number of accurate photos in the past, but typically obtains the photos by scouring web postings from Asia and thus the photos typically come from a variety of sources that make it difficult to determine their reliability.
Edge view of claimed iPhone 5S and iPhone 6 speaker enclosures