Return the begin and end indices w.r.t. ``self.__mfcc_mask_map``,
        corresponding to indices in the MIDDLE portion of the wave,
        that is, which fall between ``self.__middle_begin`` and
        ``self.__middle_end`` in ``self.__mfcc``.

        :rtype: (int, int)

def _masked_middle_begin_end(self):
        """
        Return the begin and end indices w.r.t. ``self.__mfcc_mask_map``,
        corresponding to indices in the MIDDLE portion of the wave,
        that is, which fall between ``self.__middle_begin`` and
        ``self.__middle_end`` in ``self.__mfcc``.

        :rtype: (int, int)
        """
        self._ensure_mfcc_mask()
        begin = numpy.searchsorted(self.__mfcc_mask_map, self.__middle_begin, side="left")
        end = numpy.searchsorted(self.__mfcc_mask_map, self.__middle_end, side="right")
        return (begin, end)