To detect stream format use:

This function will try to determine file format by parsing file name and using filename extension. If fails to determine file format using filename extension, function will try to parse file data, searching for some known headers in file. If this also fails, function will return sfUnknown. 

This function is useful if you use memory stream based on disk file because you need to specify stream format when you try to open memory stream. 

You don't need to specify stream format if you try to open disk file because you can use sfAutodetect format. 

Open file, memory stream or wave-in