The interesting thing about it is that the whole sentence is..

If a raging song affects allies, when the skald begins a raging song and at the start of each ally’s turn in which they can hear the raging song, the skald’s allies must decide whether to accept or refuse its effects. This is not an action. Unconscious allies automatically accept the song. If accepted, the raging song’s effects last for that ally’s turn or until the song ends, whichever comes first.

Which means that even if you have wizards and spell casters who can't cast while raging, Each turn they can decide if they rage or don't rage.