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