Unity doesn't work with Microsoft cognitive SpeechToText iOS SDK

I am using Unity 5.3 and the latest MS STT iOS SDK. If I set UnitySetAudioSessionActive(1), the MS STT will give me an error status - SpeechClientStatus_MicrophoneUnavailable at the second time when I invoke the speech recognition.

The reason that I had to call the UnitySetAudioSessionActive(1) is that after I use the MS speechToText, my IOS application can't play a audio clip without any error. This is the only way I found that I can hear the audio clip again.