The say Command is not Working in High Sierra
Bug?

I could tell something was wrong because the Speech section of the Accessability pane in System Preferences was stuck loading voices and the voice sample play button also did not work. I killed some speech and audio-related processes with some luck.

I was also able to reproduce the hang by using the -o option of the terminal say command.

Workaround without reboot:

Try killing speechsynthesisd after finding it with

ps ax |grep speechsynthesisd


or just kill it this way:

kill `pgrep speechsynthesisd`