You can capture raw audio input from the microphone by using the getUserMedia() API. https://developer.mozilla.org/en-US/docs/Web/API/Navigator/getUserMedia
This API is still in development and not supported by all browsers, and there is no standardised version yet. For now there is no support for Safari (iOS). The best support is for Chrome and Firefox browsers.
For more information check the Mozalla Developer Network articles:
Hopefully, in the near future all modern browsers support this API. But currently, there are no updates since July, 2016.