UPDATE:
The problem was solved! See the first comment.
I need to detect the keys pressed in the wav file using the FFT. But I'm completely stuck with it, just don't know what to do. I have read that you can detect the tones with the help of Goertzel algorythm, but, again, I haven't found nor explanation for it nor any example of working code. With audioread I got the wave and created the plot of it, but what should I do next to identify the frequencies for each key and filnally the keys?
Can someone please share with me an algorithm how to solve this problem?