I'm betting that DCOP uses X server commands to connect and communicate. In that case, even your own user running under cron may not have authority to connect due to lack of X authority. The easiest way around that that I know of is to actually run under X. Try using kcron - I'm not sure, but it may run under X, which would give anything it runs the authority to communicate with the local X server.

(I'm guessing that amaroK is a KDE application, that's why I'm suggesting kcron. If you're running gnome, you may want to look for a gnome equivalent, although kcron should work just as well under gnome as it does under kde: it should either work in both places or neither.)