By using the open SNDFILE with a file descriptor function:
SNDFILE* sf_open_fd (int fd, int mode, SF_INFO *sfinfo, int close_desc) ;
it is possible to open sound files embedded within larger files. There are however a couple of caveats:
The test program multi_file_test.c in the tests/ directory of the source code tarball shows how this functionality is used to read and write embedded files.