managed to get Asterisk to call tts_callback and cereproc chris to speak – here is the dialplan code

; cereproc application tts_callback refrences audio.voice and license.lic to read exten.xml and output tts_callback.wav

exten => _1XX,n,System(python /home/chris/Dropbox/K6_primo/cerevoice_sdk/examples/python/tts_callback18.py -L /home/chris/Dropbox/K6_primo/cerevoice_sdk/examples/basictts/license.lic -V /home/chris/Dropbox/K6_primo/cerevoice_sdk/audio.voice  /home/chris/Dropbox/K6/${EXTEN}.xml) 

; convert wav file format to sln file

exten => _1XX,n,System(sox /${EXTEN}.wav -t raw -r 8000 -s -2 -c 1 /home/chris/Dropbox/K6_primo/sounds/chris/render/tts_callback.sln)

; play TTS output using playback command

exten => _1XX,n,Playback(/home/chris/Dropbox/K6_primo/sounds/chris/render/tts_callback)