Try morphological dilation and erosion - they're the local max and min. Use imdilate and imerode if you have the image processing toolbox.
numberOfSamples = 200;
t = linspace(0, 4*pi, numberOfSamples);
decay = exp(-t);
y = 10 .* sin(t) .* exp(-t) + rand(1, numberOfSamples);
plot(t, y, 'r-');
grid on;
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
upperEnvelope = imdilate(y, true(1, 9));
lowerEnvelope = imerode(y, true(1, 9));
hold on;
plot(t, upperEnvelope, 'b-', 'LineWidth', 2);
plot(t, lowerEnvelope, 'b-', 'LineWidth', 2);
Or try this File Exchange submission: http://www.mathworks.com/matlabcentral/fileexchange/27662-evolve-top-and-bottom-envelopes-for-time-signals-i-e
