Same happens to me on my iBook 14''.
Go to spotlight, and type in "midi"- go to it. Mke sure the sliders at the bottom right are equal. If they are, and the problem persists, restart iTunes.
Sometimes when I play music from iTunes only one of my speakers plays music, does anyone know why this happens??
PS
iBook G4
1.42ghz
1gb ram
super drive
airport extreme and bluetooth

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