混合音频信号时显示Inner matrix dimensions must agree

[s1,fs1]= audioread('est1.wav');
c1=s1(:,1);
c1=c1/max(abs(c1));
[s2,fs2]= audioread('est2.wav');
c2=s2(:,1);
c2=c2/max(abs(c2));
[s3,fs3]= audioread('est3.wav');
c3=s3(:,1);
c3=c3/max(abs(c3));
s=[s1;s2;s3];
a=rand(3,3);
x=s*a;
x1=x(1,:);
x2=x(2,:);
x3=x(3,:);
figure(4)
subplot(3,1,1);plot(x1);axis tight;%title('混合以后的观测信号x1');
subplot(3,1,2);plot(x2);axis tight;%title('混合以后的观测信号x2');
subplot(3,1,3);plot(x3);axis tight;%title('混合以后的观测信号x3');