It hurts me to advocate Windows but using tcom with the free speech engine from Microsoft is almost too easy (yes I know Microsoft and free speech sounds like an oxymorin :)

 package require tcom
 
 set voice [::tcom::ref createobject Sapi.SpVoice]
 
 $voice Speak "Hello World" 1
 
 after 3000
 exit

The speech SDK is available from http://www.microsoft.com/speech/download/sdk51/ (51MB), there is a beta of .Net Speech at http://www.microsoft.com/speech/ (200MB) but I haven't tried that yet. - VPT

[MG] Anyone happen to have a link for more info on this? I've been using it successfully for a couple of weeks in something, but someone just found an error - 

  $voice Speak "<> test <" 1

returns "0x80045042 {Unknown error}". I did a search on the Microsoft website for the code which, naturally, came up blank. Any ideas would be appreciated :) 

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[GPS] [Rsynth] is a nice public domain package that I've used for speech synthesis.  I wrote a say_this.tcl script that built a GUI for tweaking the voice.  I've been thinking about improving Rsynth, because it seems to be at the moment dead.  Another tool that I've heard good things about was Festival[http://www.cstr.ed.ac.uk/projects/festival/].  CMU's Sphinx[http://fife.speech.cs.cmu.edu/sphinx/] is another tool that may be good, but I haven't heard from users of it.

See [Festtcl] for a Tcl interface to [Festival].

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Tcl'ers may be interested in the [CSLU Toolkit] [http://cslu.cse.ogi.edu/toolkit/] developed at the Oregon Graduate Institute's Center for Spoken Language Understanding.  It includes a [RAD] environment which supports Tcl and provides tools to do [Speech Recognition] as well as speech synthesis. -- [aricb]
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JKM would like to know if [MG] found any solution to his "0x80045042 {Unknown error}".  I'm getting the same error, but it may be for a different reason.  By default, the SAPI tries to interpret the string with XML tags if the first character is '<'.  Change your 1 to a 17 and you should be alright.  I'm getting the same error with 

  $voice Speak "c:/code/tcl/SAPI2.txt" 5

any help would be appreciated.