It’s quite clear from the Apple documentation that this is possible. As is their wont, they have pages and pages of documentation saying what is possible, but not a single piece of code. And in this case, there is one crucial piece of info missing from their documentation.
via Anna Callahan » Blog Archive » Duck and Resume: a little love from the iPhone music player.
Great post about how to have music fade some, but not completely, when playing another audio file.