sr = 44100; % the sample rate in Hz
d = .5;
f_do = 261.6;
f_re = 293.6;
f_mi = 329.6;
t = linspace(0, d, sr*d); % a time vector
do = sin(2*pi*f_do*t);
re = sin(2*pi*f_re*t);
mi = sin(2*pi*f_mi*t);
silence = zeros(1, sr*d);
soundsc([do, re, mi, silence, do], sr);