Put the voice over in a new audio track.
okay, but how to reduce the original volume for 21 clips at a time?
That is done automatically.
Use the Mixer to attentuate the level on any track you wish.
In PPRO you can control levels at a clip level ( each individual clip) or at track level by using the Mixer.
As has been mentioned, there are basically three ways to adjust the Levels of Audio in PrPro:
At the Track Level with Track Volume Keyframes, using the Audio Mixer (the method that I'd select here)
By Keyframing Volume per each Clip (one could then Copy/Paste Attributes for others)
By Rt-Clicking on the Audio and choosing Gain (not my recommended solution)
PS - if you have not used the Audio Mixer, I would definitely read up on it first. It is not difficult, but there are some elements, that you will want to be familiar with. Also, if doing Track Keyframes with the Automated Sends, you might want to adjust the Keyframing frequency in Edit>Preferences, as I find the default to produce too many Keyframes - might work fine for you however.