These voices come from:
http://www.festvox.org/flite/packed/flite-2.1/voices/
To install them, for instance, as root:
mkdir -p /usr/share/flite/voices
cp cmu_us_aew.flitevox /usr/share/flite/voices/aew

The voice name, in this case aew, should exists in
the file/etc/speech-dispatcher-modules/flite-generic.conf
for this file to be available in Orca and other
applicationd linked to Speech Dispatcher.