How to decode headphone mic audio signal into digital in android?

i am working over a project where i want to use some external sensors with 3.5m jack so firstly i am tring to get headphone mic signal and convert it into digital. i tring manny things but not able plz help

 mRecorder = new MediaRecorder();
        mRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
        mRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
        mRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
                mRecorder.setOutputFile("/dev/null"); 



int AUDIO_BUFFER_SIZE = ACQ_AUDIO_BUFFER_SIZE; //44000;//200000;// 16000;
    int minBufferSize = AudioTrack.getMinBufferSize(AUDIO_SAMPLE_FREQ, 2, AudioFormat.ENCODING_PCM_16BIT);
    if (AUDIO_BUFFER_SIZE < minBufferSize) AUDIO_BUFFER_SIZE = minBufferSize;

    Log.i(TAG, "buffer size:" + AUDIO_BUFFER_SIZE);
    byte[] audioData = new byte[AUDIO_BUFFER_SIZE];

    AudioRecord aR = new AudioRecord(MediaRecorder.AudioSource.MIC,
            AUDIO_SAMPLE_FREQ, 2, AudioFormat.ENCODING_PCM_16BIT,
            AUDIO_BUFFER_SIZE);