I had a similar issue. Below are the steps that solved it for me. More info at Re: Siri not working on Sierra.
Siri appears to be working now. Here's what I did...
- I unplugged my OWC dock from my MacBook, and unplugged all accessory cables from my OWC dock.
- I then plugged in my OWC dock to my MacBook (no accessories plugged into OWC dock).
Siri worked.
- I then proceeded to plug each accessory into the OWC dock, and tested Siri after each step.
Siri would stop working after plugging in a certain accessory.
- Each time Siri stopped working I started over (back to step 1).
- After the third time (or so) of Siri not working after a certain point, I turned Siri off and on again via System Preferences.
After doing this, Siri has now been working consistently.
I don't know if the key was unplugging/plugging in the dock/accessories, or if the solution was turning Siri on/off (I had previously tried turning Siri on/off but it didn't fix it).
I've shut-down and restarted several times now and Siri continues to work without any interaction from me.