As a customer using the digital output of an Apple Airport Express into our products you may have experienced some unexpected noises while listening to music.
After extensive investigation we have discovered that the digital output of the Airport Express is actually remarkably unstable with a very high level of Jitter (for more information on this topic see this article http://www.stereophile.com/reference/1093jitter).
While we do our very best to mitigate for jitter present in the digital signal, our systems can only tolerate this error to a certain extent. In order to increase our tolerance we would have to reduce the level of accuracy in our signal processing.
Our advice if you do experience this problem with your Airport Express is to use the analogue audio output.