Adjusting The Sound's Volume
In addition to setting the track volume, you can also set the volume of individual sounds.

To edit the volume of a sound, go to the toolbar and make sure that Volume is selected.

When Volume is selected, each sound clip shows its own volume envelope.  In the sound below, the volume envelope corresponds to the two green points and the line connecting them.

This shows a clip with two envelope points, each at 100% volume. 

Change the Volume Envelope
Click on the line to create a new volume point.  The cursor will change to an Envelope Point Edit cursor.

In the image above, a new volume envelope point has been created.  Click on the point to drag it to a new volume level.

In the image above, the volume of the sound now goes from 100% up to about 170% and then slowly down to 100% again.  In this manner, you can add as many points as you'd like to shape the volume of the sound over time.

Volume Range
Volume can be set to values from 0% to 200%  ( -Inf dB to +6 dB ).  For better precision while dragging envelope points, you can resize the track.

Keyboard Modifiers
Hold the Ctrl key down and to add points anywhere on the sound. 
Hold the Shift key down and click down on an envelope line to move the line up or down.
Hold the Alt key down and click points to remove or delete them.

Setting Exact Volume
If the desired level cannot be set via mouse dragging, you can set it by right clicking on the point and choosing Edit Exact Value...  This brings up a dialog box that lets you set the exact value.

Fades, Boosts and Reductions
These are shortcuts that add multiple envelope points at a time.  Select an area of a sound and choose Fade In, Fade Out, Reduce, or Boost from the Sound menu.  Fades go from one volume to another over time.  Boost increases the volume for a selected period of time.  Reductions lower the volume for a selected period of time.  You can Normalize a sound by selecting it and then choosing Normalize from the Sound menu.

For tips on boosting or reducing volume, see How To Boost Or Reduce Parts Of Sounds.
For tips on creating fade ins and fade outs, see How To Add Volume And Pan Fades On Sounds.