Do FFT to audio signal that record by using arduino

Haii. I am using arduino nano to record audio and save it in .wav format file. All I know is arduino only show positif amplitudo signal like the picture below, and it doesn't like the other audio signal that has positif and negatif value. Can I use the signal to find the frequency domain using FFT? Or is there any ways to prosess the signal so it can looks like the other audio signal?

Can anyone please tell me how to solve it, Thank you

This is the code that i used :

clc;

close all;

clear all;

[audio, fs] = audioread('3.wav');

x = (audio+1);

subplot(2,1,1);

plot(x);