Chrome OS comes with a speech synthesis engine developed internally at Google called PATTS. It's based on the same engine that ships with all Android devices.
This is for Googlers only.
Visit http://go/chrome-tts-blaze for instructions on how to build the engine from source and get the latest voice files.
When debugging, start Chrome from the command-line and set the NACL_PLUGIN_DEBUG environment variable to 1 to print log messages to stdout.
First, follow the public Chromium OS Developer Guide to check out the source. At a minimum you'll need to create a chroot. You do not need to build everything from source. You do need to start the devserver.
Next, flash your device to a very recent test build. Internally at Google you can do this with the following command when the dev server is running, where CHROMEBOOK_IP_ADDRESS is the IP address of your Chromebook already in developer mode, and $BOARD is your Chromebook's board name.
cros flash ssh://CHROMEBOOK_IP_ADDRESS xbuddy://remote/$BOARD/latest-dev/test
Before you can make changes to PATTS, the first thing you need to run (from the chroot) is call cros_workon with two relevant ebuilds:
cros_workon --board=$BOARD start chromiumos-assets cros_workon --board=$BOARD start common-assets
From outside the root, from anywhere under your top-level <repo-dir>, pull down the relevant sources:
repo sync
Again, outside the root, make sure you're in the <repo-dir>/src/platform/assets directory and run repo start to create a branch:
cd src/platform/assets repo start <branch_name> .
The PATTS data files can be found in this directory:
src/platform/assets/speech_synthesis/patts
When updating the files, the native client files (nexe) need to be zipped.
Replace all of the files you need to update, commit them using git, then from the chroot, run:
emerge-$BOARD common-assets cros deploy CHROMEBOOK_IP_ADDRESS common-assets
Note that you need to call cros_workon on both chromeos-assets and common-assets. You will be changing files in chromeos-assets, but to flash it onto your device, you need to emerge and deploy common-assets.
After that, reboot your Chromebook and verify that speech works.
To upload the change, use repo upload, something like this:
git commit -a BUG=chromium:12345 TEST=Write what you tested here repo upload .
Note that sometimes you'll have to update the ebuild file that takes the patts data files and installs them, unzipping the .nexe files in the process.
For example, you'll need to edit the ebuild if you add or remove a language code, or if you add or remove a file that needs to be installed as part of the extension.
To update the ebuild, edit this file:
/third_party/chromiumos-overlay/chromeos-base/common-assets/common-assets-9999.ebuild
If you need to land changes to both common-assets and chromiumos-assets, upload the changes separately and then make them depend on one another using this syntax in the changelog:
CQ-DEPEND=CL:12345
Note that you can (and often should) have two changes depend on one another so they'll land atomically.