KEAKA PowerController will wake your computer from sleep and then play an iTunes alarm, but it isn't designed to boot up your computer and then play the alarm.
Due to a slight difference in iMac G5 machines, PowerController will boot iMac G5s. However, by the time the computer finally finishes booting up, the time for the alarm has passed, and thus PowerController will not play iTunes. This sounds like the situation you are seeing. To work around this, you can use PowerController to set a "wake" event for 3 minutes before your iTunes alarm, which will allow your iMac G5 to finish booting in time for the iTunes alarm.
In a future version of PowerController I will try to add full support for booting the computer (all models, not just iMac G5s).
I will also try to add an option to customize the speed at which the volume increases.
Keaka Jackson
(Version 2.0)