Animation or sound is often used in user interfaces as an attempt to improve users' perception and comprehension of evolving situations and support them in decision-making. However, empirical data establishing their real effectiveness on the comprehension of changes are still lacking. We have carried out an experiment using four combinations of visual and auditory feedback in a split attention task. The results not only confirm that such feedback improves the perception of changes, but they also demonstrate that animation and sound used alone or combined bring major improvements on the comprehension of a changing situation. Based on these results, we propose design guidelines about the most efficient combinations to be used in user interfaces.