If you don't want to cut up the clips, you can just click on the volume line a couple of times to make four dots, and drag the dots in the middle down to change the level of the section you want to adjust, like this:
To cut up clips, so you don't affect the volume anywhere else in the video, select a starting point and press ⌘T to make a cut. Press ⌘T again at an end point, and you can adjust the volume of the smaller clip you just made:
