click on audio, press shift v, right click on blue center line to add a point (left click)where you want to start to change volume, click one near that where you want to end volume change, and do the same thing at the end where you want the volume back where you want it. You need two points for your transition to maintain normal volume for the rest of the track. Once you have your points just drag down the section you want "quiet" (left click , hold and drag line). you can slide the points around once they are there to suit your needs