Rsynth is a speech synthesis program that runs on Unix-like systems.  It's public domain software as far as I know, because it's based on public domain code according to the README and lack of a license.  It's relatively small compared to other systems (only about 5k LOC).

BSD ports/package systems have rsynth.

Version 2.0 sources can be found here:

http://www.speech.cs.cmu.edu/comp.speech/Section5/Synth/rsynth.html

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[Category Speech Synthesis]