Just to add my experience, the demo worked perfectly straight away (Chrome 70 on macOS). Really cool to see this tech, hope it's easy to integrate.
Take a look at this standalone demo if you like https://github.com/Picovoice/Porcupine/tree/master/demo/js
it should be a matter of tweaking a few parameters.
Otherwise, I am thinking it could be something related to the microphone?