How to store the signal in a variable and plot the signal in MATLAB
I suggest you to do something like (one plot with two curves):

figure
plot(sound_L(:,1), 'b'); % blue
hold on; grid on; % prevent from redrawing and add grid\
plot(sound_R(:,2), 'r'); % red
legend({'left', 'right'});


Or if you want two subplots:

figure
subplot(211);
plot(sound_L(:,1), 'b'); grid on;
subplot(212);
plot(sound_R(:,2), 'r'); grid on;


Please mind that in your case sound_L and sound_R are the same. You are not making any use of beep variable.