Which service it is using

Hi @ugintl
It's doesn't use any backend service. We used browser SpeechRecognition API for this module. In feature we could add external service like AWS, OpenAI with help of AI module.

What is its accuracy?

That accuracy basedon end-user device specification.
More Info: https://www.learningapps.co.uk/moodle/xertetoolkits/play.php?template_id=1278#page1