Here is a list of all documented files with brief descriptions:

speex.h	Describes the different modes of the codec
speex_bits.h	Handles bit packing/unpacking
speex_buffer.h
speex_callbacks.h	Describes callback handling and in-band signalling
speex_echo.h	Echo cancellation
speex_header.h	Describes the Speex header
speex_jitter.h	Adaptive jitter buffer for Speex
speex_preprocess.h	Speex preprocessor. The preprocess can do noise suppression, residual echo suppression (after using the echo canceller), automatic gain control (AGC) and voice activity detection (VAD)
speex_resampler.h
speex_stereo.h	Describes the handling for intensity stereo
speex_types.h	Speex types