Back up the entire project first, just in case.
Individual clips can be modified by clicking their FX button and choosing the NORMALIZE or AMPLIFY tool.
Multiple clips might be handled by selecting (highlighting) them, pressing <ctrl-c> to copy, then clicking the + sign next to the sequence tab, to open a blank sequence.
In that new sequence, press <ctrl-v>, to paste. Now export the sequence as an Mp4.
Bring the Mp4 back into VP on an overlay (higher numbered) track, lining up the clips it contains with the originals, which can be deleted or muted.
The new audio is now a single clip that can be NORMALIZED in VP or run through Wavepad prior to VP import.
When done, unlink video from audio and delete any confusing blank video. To avoid messing up other tracks, lock them.
If you make a mistake, immediately press <ctrl-Z> to undo.