Put your pointer over the audio track that you would like to adjust, right click, select "Insert/remove Envelope", select "Volume". That will put a blue line across the track. At places where you want to change the volume right click on the line, select "Add point", then left click the newly created point and slide it up or down till you get the desired volume label. Keep adding points and adjusting the all along the line.

You may also want to consider "normalizing" the volume in each clip. Right click on a clip, go to "Switches", and select "Normalize". This function will adjust the volume of the clip based on the highest peak in the visible clip. Often this is all you need to do to get various clips to have similar volume levels.