The core audio driver is a single-client driver. Meaning that only one app can output using the digi hardware at a time. And I believe the Finder is permanently prevented from being able to use it (otherwise, since it launches before any other app, once it acquired the hardware, no one else would be able to use it).
So if you launch iTunes and it grabs the hardware, no one else will be able to use it until you quit iTunes. And even then it's a little finicky.