Even after trying the suggested actions above I cannot switch to Alex or to any other voice but the enhanced 156MB high-quality U.S. English voice.
Alex Voice Fails to Automatically Enable After First Being Downloaded
Upon first downloading the Alex TTS Voice, VoiceOver fails to switch automatically from the voice in use while the Alex download was in progress.
Bug First Encountered
Device(s) bug has been encountered on
How often the bug occurs
Disable VoiceOver, then re-enable it. If no success, switch away from and back to the Alex voice and restart VoiceOver again.
I got some sighted help to deselect and re-select the Alex voice while VO was disabled/off. After restarting VO Alex began speaking.