I’m also not clear on the role of Audio Hijack: it’s interface is exactly the same as the Effects in NiceCast.

For the show I did Friday I set my NiceCast source as my input source and used the Applucation Mixer in the effects window to cross fade from voice to iTunes (I bet it would work the same to set the source to iTunes and use the voice over effect). I also added a VST effect to distort the audio at the end, but turned it off until needed via the Bypass button,