When I plug external mic, it forces to output input audio data

When I insert external mic, input audio comes from iPhone’s speaker, and horrible feedback happens. (When I use iPhone’s internal mic, there’s no feedback.)
How can I turn it off?

I tried to turn off output from AudioDeviceManagerComponent (there’s no sound when I tap test sound button), and in all my AudioIODeviceCallback::audioDeviceIOCallback() end with…

for (int i = 0; i < numOutputChannels; ++i)
    if (outputChannelData[i] != nullptr)
        juce::FloatVectorOperations::clear(outputChannelData[i], numSamples);

But it didn’t go well.

I’m using iPhone6 and Shure MV88 for the external mic.
Unfortunately I don’t have any other external mic for iPhone so I don’t know it’s due to MV88 or JUCE.
So let me know if you know what’s happening.