After ripping to MP3 format, I have been using MP3Gain to volume level. The changes it applies to MP3 files can be undone as they do not involve re-encoding of the music content in the file, just the addition of header information.
It can be used to process files on any media that allows the individual files to be rewritten.
That being said, what hamluis describes as far as storage is also precisely what I do. All of my music ripping takes place initially on my PC and I immediately apply MP3Gain to all files I've just ripped before I move them to their permanent home in my music library. This makes it very easy to grab random mixes of albums, songs, etc., out of my library on to any portable media of my choosing and having the knowledge that all material has been volume leveled such that I don't have to constantly be fiddling with the volume controls as the songs change.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Presenting the willfully ignorant with facts is the very definition of casting pearls before swine.
~ Brian Vogel