First... this was the wrong approach to take for solving this problem... as you probably figured out even if you did get the greetings and voice names higher the prompts would not be and you'd have a pretty uneven sounding solution. The key here is to have all prompts, voice names and new recordings coming in at the same level. If it's too quiet playing back you can boost the playback volume a little for _all_ outgoing plays using the WAVGain utility. Having various sources of files having different base volumes is not an approach that is going to make the customer happy I can assure you.
That said, it took 20 minutes the first time since it had to open each WAV file and fiddle with it to get the volume to reset. The restore simply does a massive file copy from the directory where you backed up the originals to the \streamfiles directory which should happen very quickly.
My guess is the volume of your prompts was not -26 to begin with - you didn't provide a Unity version for me to work with here so I'm just flying in the dark but this would be my first guess.
