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

io
sink.h	Media sink to write blocks of consecutive audio samples to file
source.h	Media source to read blocks of consecutive audio samples from file
notes
notes.h	Note detection object
onset
onset.h	Onset detection object
pitch
pitch.h	Pitch detection object
spectral
awhitening.h	Spectral adaptive whitening
dct.h	Discrete Cosine Transform
fft.h	Fast Fourier Transform
filterbank.h	Filterbank object
filterbank_mel.h	Filterbank object coefficients initialization
mfcc.h	Mel-Frequency Cepstrum Coefficients object
phasevoc.h	Phase vocoder object
specdesc.h	Spectral description functions
tss.h	Transient / Steady-state Separation (TSS)
synth
sampler.h	Load and play sound files
wavetable.h	Wavetable synthesis
tempo
tempo.h	Tempo detection object
temporal
a_weighting.h	A-weighting filter coefficients
biquad.h	Second order Infinite Impulse Response filter
c_weighting.h	C-weighting filter coefficients
filter.h	Digital filter
resampler.h	Resampling object
utils
log.h	Logging features
parameter.h	Parameter with linear interpolation
aubio.h	Global aubio include file
cvec.h	Vector of complex-valued data, stored in polar coordinates
fmat.h	Matrix of real valued data
fvec.h	Vector of real-valued data
lvec.h	Vector of real-valued data in double precision
musicutils.h	Various functions useful in audio signal processing
types.h	Definition of data types used in aubio
vecutils.h	Utility functions for fvec_t