trace("Available sound input devices:");

//Detect which Microphone Devices are available
var deviceArray:Array = Microphone.names;
for (var i:int = 0; i < deviceArray.length; i++)
{
	trace(" " + deviceArray[i]);
}

//Create a new Microphone
var mic:Microphone = Microphone.getMicrophone();
//Default = 50
mic.gain = 60;
//22Kbs
mic.rate = 22;
//Set this to true if you're not using a Headset
mic.setUseEchoSuppression(true);
//Routes the Audio Captured back to the Local Speakers
mic.setLoopBack(true);
//Level of Noise Supression
mic.setSilenceLevel(2, 1000);

mic.addEventListener( ActivityEvent.ACTIVITY, this.onMicActivity);
mic.addEventListener( StatusEvent.STATUS, this.onMicStatus);

var micDetails:String = "Sound input device name: " + mic.name + '\n';
	micDetails += "Gain: " + mic.gain + '\n';
	micDetails += "Rate: " + mic.rate + " kHz" + '\n';
	micDetails += "Muted: " + mic.muted + '\n';
	micDetails += "Silence level: " + mic.silenceLevel + '\n';
	micDetails += "Silence timeout: " + mic.silenceTimeout + '\n';
	micDetails += "Echo suppression: " + mic.useEchoSuppression + '\n';
	micText1.text = micDetails;

function onMicActivity(evt:ActivityEvent):void
{
	trace("activating=" + evt.activating + ", activityLevel=" + mic.activityLevel);
	meter_mc.mask_mc.scaleX = mic.activityLevel / 100;
}

function onMicStatus(evt:StatusEvent):void
{
	trace("status: level=" + evt.level + ", code=" + evt.code);
}

Here's an example of how to use the Microphone API in Flash

Flash: Microphone

ActionScript 3