Use the audio envelopes.

Make sure that audio envelopes are enabled in your drop-down menus. On your audio track you'll see wave graph of the sounds and, running down the center length, a line. Double-click on this line and it gives you a control point. Make a couple control points and, by setting them higher or lower, you'll control the sound level for exact points on this audio track.

BTW, if you're going to move this clip later, make sure you lock you audio envelopes to your audio tracks or these points will stay in place when your clips move.