I can imagine that's frustrating.
Yet the word 'permanently' is pretty descriptive. And it is unticked by default for a reason.
Since you chose 'permanently', your audio files have been re-written at a lower volume.
"restore volume' only removes any written RG 'indicator' tags.
This will not do anything to the actual audio file, and so it won't help your case.
One option you could choose, is to normalize again 'permanently', but now with opposite values. Thus making them louder again.
Of course this will (again) have some possible effects on the sound quality of the files.
A good advice for anybody, if you are not familiar with these kind of options, experiment with some back-up files first before using them on a precious library.