There is a volume slider in the track header on the left edge of the screen. You can use that to raise the volume of the entire track up to +12dB.

If the audio clip itself is very low volume throughout, with no loud peaks, you can normalize it. Right-mouse-button click on the audio and from the popup menu choose Switches / Normalize. This will raise the volume of that clip so that the loudest peak becomes 0dB and the rest is raised with it.