Without the data, we can only guess. And the most probable guess is that y1 is a 2D array and using length() with non-vector array results in ambiguous outputs. A better option is to use size().
Try this -
y1 = audioread('bel.wav');
var = 0.1;
N = size(y1,1);
noise_1 = var.*randn(N,1);
y_1n = y1 + noise_1;
subplot(311)
plot(y_1n)
