Here's one way to do this. I'm showing two graphs, one with 5 Hz amplitude modulation, one with 5 kHz amplitude modulation. I've also passed the modulated signal into the sound function, so you can hear the result as well. At 5 Hz, the effect in music is known as tremolo.
duration = 3; % seconds
samplingRate = 8192; % per second
samplingInterval = 1 / samplingRate;
t = 1:samplingInterval:duration;
y1 = cos(2*pi*300*t); % signal
y2 = cos(2*pi*5*t); % modulating signal (frequency = 5 Hz, adjust as desired)
y2 = rescale(y2,0.25,1); % modulation amplitude min/max factor
y = y1 .* y2; % create amplitude-modulated signal
% plot about 1 second of the signal (to show amplitude modulation)
plot(y(1:9000))
set(gca,'ylim', [-1.2, 1.2]);
% play it
sound(y, samplingRate); % NOTE: sRate not needed if 8192 (default)
5 Hz modulation:
5 kHz modulation:
