CHUNKSIZE too small
according to the spec, a vorbis page may be 255*255+header bytes big. _get_prev_page() only seeks backwards by CHUNKSIZE bytes, where CHUNKSIZE is defined to be 8500 bytes. that means, _get_prev_page() may fail to find the previous page eventhough it's operation on a spec conforming ogg stream. though it's prolly inefficient for _get_prev_page() to seek back by ~64k by default, it should be able to revert to that much backstepping if nothing is found in the first 8500, so as to work correctly on conforming ogg streams.