select the bars in the playlist in which you want to manipulate the volume, then create an automation clip for the control. It will be as long as the bars you selected. Now with your volume automation clip make sure that the beginning of the clip is the same level as your sample was before, create the automation and end with two points above each other. the last one added will be the level for then on , if it is the level that the sample was before, you'll have reached what you wanted to do.
like this
Hope I understood your question right