You can normalize the volume on individual clips by opening the Inspector. Click on the Audio tab then click on Normalize Clip Volume (near the bottom of the Inspector panel).
To open the Inspector, either double-click on the clip or click on the gear icon at the start of the clip and select Clip, Video or Audio Adjustments. Alternatively, press the I key on the keyboard (I for Inspector) after selecting the clip, or click on the circled i icon in the centre tool bar.
You can follow the same process for the other clips in the project. iMovie Help explains the Normalize function as follows:
"Audio normalization is a fast way to adjust clip volume to optimal listening levels. Normalization calculates the volume change required to bring the loudest part of the selected audio clip to maximum listening levels without distortion. After this amount is calculated, the volume of the rest of the audio clip is adjusted by the same amount."
Another method to apply the audio changes to all other clips in the project is to copy the adjustments to the other clips - search on this topic in iMovie Help: Apply the same adjustments to multiple clips
NOTE: If you can't locate the Help topic I mentioned, see this link to the Apple article:
Message was edited by: John Cogdell - added NOTE