Como aplicar a FFT em um áudio gravado pelo microfone do smartphone?

I have a code in Android Studio (java language) that uses the microphone from smartphone, records an audio, and then replay it for the user (using the MediaRecorder class). This audio is being saved as storage/emulared/0/....audio_record.3gp (I confirmed it using the EX File Manager app). Now, I need to use the FFT code (such as this one: http://www.ee.columbia.edu/~ronw/code/MEAPsoft/doc/html/FFT_8java-source.html) to pick this audio recorded, and show the user the audio spectrum. The main of the app that I''m creating is to calculates and displays the FFT of audio from the microphone.

How can I use the FFT to this application? How to pick this audio recorded and apply the FFT code?

Thank you!