HELP! FFT and I2S

[Grumpy_Mike]

@gleonn

#define SAMPLING_FREQUENCY 2000 //Hz, must be less than 10000 due to ADC

But you are not using the ADC. You have setup the I2C for

@gleonn

.sample_rate = 44100

I am not sure if the way you put the I2S samples into the FFT buffer will work. So I would test this by sending the FFT buffer to the serial plotter to see if you are getting a waveform, before worrying about what the FFT produces.