By default, the volume slider in VLC allows you to increase the volume up to 125%. If you have a file with really low, inaudible audio, you may want to boost the volume above 125%.
Increasing audio volume above 100% percent causes audio distortion; it may harm your audio equipment; extremely loud sounds could even cause hearing damage. Prolonged exposure to moderately loud sounds also causes hearing damage. Use this option only for files whose volume is very low.
Increasing volume up to 200% using the Keyboard
Using the keyboard shortcut, the volume can be increased up to 200% without having to change any VLC setting. The shortcut to increase volume is CTRL+UP To decrease volume use CTRL+DOWN
Changing the volume slider maximum
As mentioned before, the maximum volume which can be achieved using the volume slider is 125%, by default. This default setting can be changed from preferences.
From the Tools menu choose preferences or press CTRL+P. Make sure that show all settings is selected.
This would change the preferences window from Simple preferences to Advanced preferences. Go to Interface ➾ Main interfaces ➾ Qt ➾ Maximum volume displayed.
Set the maximum volume displayed to the desired percentage.