Select the audio track you want and type V. This brings up a blue line that represents the volume (a volume envelope). If you double click on the line you add an anchor point, which you can slide along the line like a bead. Try adding 4 anchor points and then grab the blue line between beads 2 and 3 - you can move it up or down to adjust the volume. You can add more points and slide them round to make all sorts of adjustments.

Ian G.