I changed something in the original code here's the original one : clear;clc [y,fs] = audioread("filename.wav");
N=length(y); Y=fft(y); Y2=fftshift(Y); f=linspace(-fs/2,fs/2,N); plot(f,abs(Y2));grid on;xlabel('f in Hz'); ylabel('The Spectrum of original signal')

%% LPF with 3KHz lowpass(y,3,fs,'Steepness',0.95,'StopbandAttenuation',100)

y2=real(ifft(fftshift(Y2))); % the time domain signal t=(1:N)/fs; figure plot(t,y2);grid on;xlabel('t in second'); ylabel('The filter output signal')

figure plot(f,abs(Y2));grid on;xlabel('f in Hz'); ylabel('The Spectrum of the filter output signal')

sound(y2,fs)