Most web browsers will not automatically let pages start detecting audio for security reasons, so when the browser wants use your microphone it must ask for permission. For Firefox the request looks something like this:
If the browser has been given permission to use the microphone, a red microphone icon will show in the browser address bar like this (in Firefox) when audio input is active:
If you opted to \"Remember this decision\" when the permission was originally requested, your choice will be saved and used next time as well, for all pages across the same website. The permission given to each website can be changed in the main settings for the browser (for Firefox type about:preferences#privacy in the browser URL).
If a site has been given permission, it will look something like this, where you can select Allow or Block.
If microphone permissions are enabled but your computer is still not detecting audio input, you could try these other sites which use HTML5 audio to see if they're ok. Other things to consider that might prevent audio input are volume settings, which input jack the mic is connected to, preamp battery state, and selection of a 'default device' when more than one microphone is present.