what happens If I change the phase and magnitude of a signal I have a speech signal. I want to know what happens when I make some changes on the phase of its frequency reaponse. When I set its phase to zero I observed that the new signal is even and the amplitude of the first sample became larger. Does it mean all the frequency components get the zero delay? But there is again amplitude in middle part of the signal. Is there any better interpretation for makeing the phase of signal zero?

When I set the frequency resposnse to the constant value of its average of magnitude what will extactly happen?

Thanks in advance for any help
Regarding the phase: if you set all frequency components to have zero phase, then the signal becomes essentially a sum of sinusoids that tends to an impulse as the frequency increases. In other words,

$$\delta(t) = \int_{-\infty}^\infty \cos(2\pi ft) df.$$

In your case, the maximum frequency is limited, which means that the signal will actually become a sinc. Try this in Matlab (or similar):

fs = 1000;
t = -0.1:1/fs:0.1;
f = 0:200;

s = zeros(1,length(t));
for ff = f
    s = s + cos(2*pi*ff*t);
end
plot(s);


and you'll get a plot like this:


When each cosine has a different amplitude, the signal still will tend to acquire a sinc-like shape.