Speech-to-Text – Asset Store

This one from Unity is free…

Requires Unity 5.4.0 or higher. Provides a wrapper for several speech-to-text solutions and a sample scene that compares text transcriptions from each API. Please checkout the README for instructions on how to install and configure required software to run the sample scene.

Source: Speech-to-Text – Asset Store

https://labs.unity.com/article/speech-recognition-and-vr

This is on Sale now… but… do I really need it?
https://www.assetstore.unity3d.com/en/#!/content/72625

https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjb8r6osJLTAhWJoRQKHSuMBKwQtwIIMTAC&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DvkO7aNM5kLE&usg=AFQjCNHTaRfDRMUtyrlJpl6zHUkWKPA3Mg&sig2=M63EW6wn9dBWuGPu7LVxZg&bvm=bv.152174688,d.bGs

http://frostweep.ru/wiki/google-cloud-speech-recognition/google-cloud-speech-recognition.html

https://forum.unity3d.com/threads/released-unity3d-google-cloud-speech-recognition-vr-ar-mobile-desktop.346148/