If people are considering doing much sound processing for VoIP,
particularly real-time, it may be worthwhile to examine the SndKit
which is part of the MusicKit (www.musickit.org). This ObjC library of
realtime sound processing and visualisation already runs on MacOS X and
GNUstep, the latter tested on both Linux and Win32. The SndKit portion
of the MusicKit does not include code from NeXT/Apple and so can be
used without license encumberance. It's only in the case of using the
music representation (notes) and synthesis parts that Apple has any
(uncontested) claim to some undefinable portion of that code. Just a
suggestion.