Simulink: Microphone input find frequency / FFT

Hi, i am totally new to matlab / simulink and i need help. I want to create a guitar tuner, but i am struggling with analyzing the input signal correctly.

i have got some code to find the frequency of a generated signal:

Fs = 96000;
t = linspace(0,1,Fs);
x = cos(2*pi*4999*t)+randn(size(t));

xdft = fft(x);
fftabs = abs(xdft(1:length(x)/2+1));
freq = 0:Fs/length(x):(Fs/2);
[~,maxindex] = max(fftabs);

mx = freq(maxindex);
fprintf('The maximum occurs at %2.1f Hz\n',mx );

this works perfectly fine, but when i replace the generated signal with the real microphone input i got some strange results.

I created a MATLAB function block with a modified version of the code above:

function res = fcn(x)
Fs = 96000;

xdft = fft(x);
fftabs = abs(xdft(1:length(x)/2+1));
freq = 0:Fs/length(x):(Fs/2);
[~,maxindex] = max(fftabs);

mx = freq(maxindex);
res = fftabs;
fprintf('The maximum occurs at %2.1f Hz\n',mx );

end

in the following picture you see my current model.

For example, when I play the sound of the note "A" (440 Hertz) into the microphone, I somehow get 653hz as result and not the expectet 440hz. Does anybody know what the problem is?

Thanks! Matthias