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I really having a hard time to tackle the following question. It is asked to load a ".mat" file using load file.mat. And the result is two variables: Fs varaible contains the sampling frequency=44100, y is a 441344x1 matrix that contains data points of this audio.

It is asked to amplitude modulate this audio by multiplying a cosine function of frequency 4 Hz. This is to micmic the tremolo effect. This corresponds to y.*cos(2*pi*4.*n)

But my question is, how should I define the length of n? If I do

n=transpose(0:1/441343:1)

The resulting audio sounds very weird. Then, how should I define n then?

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Your modulation signal must be

$$m(n)=\cos(2\pi f/f_sn)$$

where $f=4\;Hz$, $f_s=44100\;Hz$ is the sampling frequency, and $n$ is the integer sample index. In Matlab this looks like

L = 441344;
n = (0:L-1);
f = 4/44100;        % normalized frequency
m = cos(2*pi*f*n);
ym = y(:).*m(:);    % modulated signal
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