“In a busy setting, do animation and sound in particular distract viewers from absorbing your messages, or do they help to draw attention? How much is too much?”
I feel animation is a better tool than sound. With most digital signage deployments, there is some sort of content loop. If sound is used, the repetition of the sound can cause the message to just become white noise. I have seen screens turned off or speakers taped over, especially in locations where the screen is installed close to a location where someone works for an extended period of time. This diminishes the effectiveness of your signage network, especially if you use the network as part of your communication plan in the case of an emergency.
Using animation can be a way to catch someone’s eye as they walk past your screen, but as with all tools, the animation needs to be used sparingly. I generally only like to see major animation at the start of a message to grab someone’s eye and then stop or at least settle for the meat of the message. If there is too much animation and moving, it can be difficult for the viewer to retain the message they are seeing.