You're not alone with this problem...
There's a lot of info available on Computer Audiophile and other websites to help troubleshooting this. Google "Apple Airport Express Streaming Issues"
Apple Airport Express to DAC
creek: It sounds like there’s an issue in using that type of cable. This is not a true way of using an optical signal. I would suggest using the jack to phono rca lead as this will use the DAC in your Creek unit rather than the built-in DAC of the Apple product.
me: I'm confused. Running optical signal to my creek DAC is wrong? You suggest running minijack to RCA on the EVO 100A amplifier? How does the EVO 50CD get the digital signal from the EVO 100A?
creek: An optical signal can only be transmitted down an optical cable that’s terminated with TOSLINK blocks at both ends.There are no digital signals past between the Evolution 50CD and Evolution 100A. The signal is only analogue; not digital between these devises. I would suggest running an RCA phono cable from the pre-amp output of your Evolution 100A and plugging that signal into the jack socket of the Airport Express.
me: but the minijack out on the airport express is a combo analog/optical output. (picture of back of airport express from apple website showing analog/optical port).