I assume that to initially get to the frequency spectrum you used the fft. To listen to it you need to convert back to the time domain with the inverse fft.
X = ifft(Y,n)
Then, if you want to listen with matlab you can use
sound(X, Fs);
where Fs is the sampling frequency of your song.
You could also export it if you want to listen outside of matlab
filename = 'song.wav';
audiowrite(filename,X,Fs);
