Sony PCG-K315B Hardware Manual page 27

Troubleshooting sound
My speakers are not producing sound
The built-in speakers may be turned off. Press <Fn>+<F2> to turn on the speakers.
The speaker volume may be turned to the minimum. Press <Fn>+<F4> to increase the volume.
If your computer is powered by battery, check that the battery is inserted properly and that it is charged.
If you are using an application that has its own volume control, check that the volume is turned up.
Check the volume controls in Windows.
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Using your VAIO computer