The 15-inch "aluminum" PowerBook G4 computers, starting with the PowerBook G4 (15-inch FW 800), have a midrange-enhancing speaker that's designed to produce a separate channel of midrange sound. The midrange speaker is located underneath and to the left of the space bar. The location of the three speakers is indicated by circles in the picture below.
There are no settings in the Sound pane of System Preferences to adjust the midrange speaker independently. Increasing and decreasing the system volume changes both the stereo speakers and the midrange speaker at the same time.
You may notice that when you move the Balance slider in Sound preferences far to the right, you can still hear sound coming from what you may perceive as the left speaker. In reality, the sound is coming from the midrange-enhancing third speaker, but because of its placement, the audio appears to be coming from the left side. This is normal.
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