Does your service work on HTTP or only on HTTPS?

Yes, the speech recognition API works extensively on the secure protocol. So your website must be served over HTTPS connection. It is good practice to implement a redirection (301 or 302) to the version with an SSL certificate, so http://example.com/page-a -> 301 -> https://example.com/page-a. That will guarantee that the voice tracker will work all of the time.