Cannot figure out why the Matrix S1 and S2 differ. Seems to work when the Matrix S1 is a random matrix (rand) but when the source is a wave file which has been made into a matrix using the Short time Fourier transform S! and S2 differ and i cannot seems to get back the original matrix S1. Anybody has any clue Why this happens?

y = wavread('source.wav');
S1= myspectrogram(y); % Generic routine to get the STFT
[U S V]=svd(S1);
S2= U*S*(V');

If anybody has any clue plz reply. Thanks for looking.

It should work. I can't see the problem...

From time to time there are some bugs appearing in Matlab...
Try to clear your workspace and try again.
Or exit from Matlab, re-enter and retry your function.

Do this to test it

clear; clc; format compact

A = [1 2 3
4 5 6
7 8 9
1 2 3
8 6 2]

[u s v] = svd(A);
m = u*s*v'

Does m equal A?