The Adjust by option will change the volume of a sound from its current setting. For example, if you adjust by 50%, the sound will become 50% louder than it is currently playing:

Use the spin buttons to increase or decrease the percentage of the sound or type the number in the box.

Note: The volume can be set to a negative number (e.g. –30%). A negative number will decrease the volume. If the current volume is 20% and you set this option to –30%, you will have effectively set the volume to 0%.