If your site is not on HTTPS :
then this is likely to be the cause of the error. Google Chrome and Mozilla Firefox both require HTTPS before they will grant access to the underlying devices.
If your site is on HTTPS:
then this error will be because the microphone/webcam was not granted permission when the little dialog showed.
This is the toolbar on Chrome when permission has been granted ...
Note the little video camera mark. It might be a microphone in some cases , but in Google Chrome at least you should see a mark there on a page with a recorder.
This is how it looks when permission has not been granted.
You can rectify this by clicking on the mark. You should then be able to grant permission to the browser to access the web cam / microphone.
(apologies because it is in Japanese, but hopefully its clear what you should do).