The organization changed from SPro 3.0. The former spro_util.* codes are now part of the libgg 2.1 library and SPro 3.1 requires that library. This library also provides an error reporting function as well as some standard error codes.
The FFT algorithm was changed to make it faster. It was previously based on Nielsen's implementation of the algorithm. The new version was adapted from the real FFT function described in the Numerical Recipes in C6. In SPro 3.0, the FFT was done on 1024 points and it is now done on 512 points. Current version of sfbank is about 5 times faster than the previous one.
The computation of the 
coefficients slightly changed at the
boundary (the boundary frames are now repeated instead of being
avoided).
Known bugs have been corrected, principally concerning the cepstral mean subtraction and error reporting is now better. Note that for a better error reporting, many function prototypes changed so that some tools developed with the previous version may be adapted before they are compatible with SPro 3.1. The order of the enumeration for the datakind_t also changed so that when using slist with old data file, the data kind printed on screen may not be the correct one.
The SPHERE compatibility is also a new feature of SPro 3.1.