A lot of digital signage applications do not use audio and indeed very many of the systems Digital View as asked for do not have audio as part of the specification. However when audio is suitable the issue is where to get it from, this is certainly a challenge for me. I quite like stock audio such as from iStockphoto and playing around with programs like Garageband (Mac only) but it seems there are other options.
One product I found that makes it about as easy as it can get is Abaltat Muse (Mac & PC), this generates audio to go with a video based on the duration of the video and the colors. Shown here is an example of a video (from iStockphoto) that I ran through Abaltat Muse: