MP3 metadata has a sort of dynamics compression hack where an individual track can say to the player “play me at 110% volume” and another track can say “play me at 90% volume” and the result is that those 2 MP3’s seem to be at the same perceived volume.
With lossless audio, you don’t have this hack. You have to use actual dynamics compression. So the way you compress the dynamics is to run the audio files through the same dynamics compressor and create new lossless audio files that all play at the same perceived volume. This is classic audio editing.
Note that this has nothing to do with normalization. You can normalize 10 WAV files and they can all still have a different perceived volume. Normalizing is math, but perceived volume comes from art. Each song has a different dynamic range because of the way the people who made it expressed themselves artistically. A dynamics compressor is how you can make 2 songs from 2 different sources match.
There is an art to finding the right settings for your dynamics compressor based on your source material. That is why the tools that set one MP3 at 110% and another at 90% do an analysis of the audio first. With lossless audio, you will listen in real-time to the output of the dynamics compressor and make the adjustments you want. Some dynamics compressors also have automatic features that simplify their use and are equivalent to what is done with MP3.
Audacity probably has a built-in dynamics compressor and if not, then you can probably use a plug-in. If you are running on a Mac there is one built-in called “AUDynamicsCompressor.”
If you were only going to have one audio processor, it would be a dynamics processor. It’s the most fundamental of audio processors. It would be hard to imagine any kind of audio editing going on without a dynamics processor.