All 17-inch MacBook Pro models sport four speakers with two separate speaker modules (left and right), each with two speakers each. Since the system can output a lot of sound, it is very important to make sure screws are properly tightened down. As a mobile product, a loose screw can eventually work itself free, and using the speakers at elevated volumes can also work toward loosening a screw that is not properly installed.
In addition, each speaker is driven by its own amplifier, as is the headphone port. As such, troubleshooting between speaker and left I/O board failures will require some part swapping to pinpoint the problem module (see the Sound heading in Troubleshooting section).
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