More handlers!  Everyone who adds sound hardware to a computer has the urge
to come up with their own file format.

More effects!  I don't know DSP at all.  An incremental FFT is the
core tool for building analog-domain effects.  Hint.  A simple
room reverb effect that does one sound bounce instead of feeding
back would also be handy, and an easy coding project.

Loop support for all formats that know about it.  WAV has
some support for loops ("cue-points") but no support for
MIDI note numbers.  Someone needs to add this in.

Comment strings.  Some file formats have space for embedded comments.
These are currently thrown away.  Printing them out, carrying them
forward, and an to add new ones would be handy.

SOX includes skeleton format files to assist you in supporting new 
formats, sound effect loops, and special-purpose programs.
The full skeleton format, skel.c, helps you write a driver 
for a new format which has data structures.  Skeleffect.c is
a starting point for writing a sound effect loop.  Sox.c is
a good starting point for new programs.  (Someone finally
did this and told me what was wrong...)

In handlers.c, note that many formats set up the header and then
use the raw driver for reading and writing.  

An effect loop for mixing mono -> stereo -> quad with sound
placement features: differential volume, phasing, and Doppler
shifting when the sound moves.  Static placement would work as
a SOX effect loop, but dynamic placement involves some scripting
feature, or joystick input etc., and should be a separate program.

Convolving filters are OK for massaging sound samples;
a much more accurate (and slower) method is also needed for
doing serious sound work.  As is, SOX is OK for PC sound 
samples, but does not make it as a CD mastering tool.

Sound needs its own compression techniques rather than text-based 
ones.  The pred effect is a framework for experimenting with
predictive/corrective compression.  Prediction via LPC
or autocorrelation or the first few derivatives
could make this a very powerful compressor.  

This software wants to be a dataflow system with signal
sources, sinks, and processors.  It wants to be class-based.
It wants to have a scripting control language.
It's really a shame I hate C++.

A pitch-shifter is badly needed.  No, just resampling isn't
good enough, because that changes the playback time.
You have to repeat little chunks or something like that.
See FFT stuff above.  The Mac SoundHack program does all
this and more.