Normalize
Use the Normalize effect to achieve the greatest amount of amplification that will not result in clipping. If you select a part of the file with the mouse, this effect will amplify the highlighted selection to the percentage of the maximum level. Otherwise the sound of the whole file will be normalized.
In the Properties Window you can change the effect properties. It's possible to select the necessary preset from the dropdown list or set a custom Normalize value. You can also tick the Bias Adjust option to adjust the addition of a high frequency signal to the audio.