a problem about getEnhancedMicrophone()

In my AIR application

code as below

step 1.

var microphone:Microphone = Microphone.getEnhancedMicrophone();

...

step 2.

var connection:NetConnection = new NetConnection();

connection.connect("rtmp://localhost:1935/emcs");

step 3.

var httpService:HTTPService =  new HTTPService();

httpService.method = "POST";

httpService.useProxy = false;

httpService.resultFormat ="xml";

httpService.url = serverUrl+"/BpMaster?id=1000";

httpService.send();

step 4.

when connection.connected==true ,i publish a NetStream to red5 server, code as below

  var nsVoice:NetStream = new NetStream(connection);

  nsVoice.client = this;

  nsVoice.attachAudio(microphone);

  nsVoice.bufferTime = 0;

  nsVoice.publish("a_test", "live");

then microphone can not be used anymore, microphone.gain==0 forever,without step3 everythine going well.

please help me solve this problem ,or explain the reason

thanks!