I'm not sure if there's a class that can exactly tell if the user has a microphone connected or not but take a look at this: http://livedocs.adobe.com/flex/3/langref/flash/system/Capabilities.html You could maybe play around with the hasAudioEncoder property and see if it helps. I think I once saw a web site that checked a microphone connection but I can't remember the name... If I find the link/name, I'll post it.
With best regards,
i tried "hasaudioencoder" property but it does not helped me. The property always returning "true" whethere I plug or unplug my mic.