Once you choose your media on the timeline that has audio, you open the properties panel (the panel on the right side of Camtasia), and then chose the audio tab at the top of the panel. When on the audio tab, you can adjust the 'Gain' slider in that panel if you wish to boost the audio of the entire clip up or down. You can also adjust the audio on the clip itself by dragging the audio line up or down. You can double click on the line to add audio points as needed and then adjust only the section you want. This is useful if you have a section that is louder or softer than another section and you want to adjust only a portion of the audio level.