title: AVAudioRecorder: can not record after other record apps
body: <p>I know this is a difficult question, because I sometimes see it, sometimes not. I use AVAudioRecorder to record audio in my application. It can run fine. But sometimes when I navigate to other apps and use microphone function of those apps, and back to my app, my record functionality can not record, all the record files have length is 0s.</p>
  
  <p>After I debug, I found out that:
  [recorder prepareToRecord] return TRUE (this method to create record file - for example .caf file)
  [recorder record] always return FALSE (this is the problem)</p>
  
  <p>I don't know why [recorder record] return FALSE. Do anyone see this problem before and can tell me how to fix this?</p>
  
  <p>Thank you very much.</p>