Welcome to the forum.
There are a couple of ways to do this:
One can Rt-click on each Clip and can adjust the Audio Gain
One can click-drag on the orange "rubberband" in the Audio Clip
One can Select the Clip and go to Effects>Edit Effects, where they can easily adjust the fixed Effect>Volume (this is the spot that I would recommend doing any Keyframing to adjust Volume up and/or down in that Clip
One can go to Window>Audio Mixer, and there adjust the Track (all Clips ON that Track) with Keyframes. Note: these are Track Keyframes, so do all editing with Clips first, because if you set Track Keyframes, and then alter the Clips, the Keyframes will remain, regardless of what you do to the Clips.