A number of songs are either really quiet or really loud, so I want to redo all volume leveling. I know it is done when songs are imported, but I can't figure out how to redo it when I want to.
It is a preference under the Playback tab. Select
You can also adjust the volume for tracks individually by choosing Get Info on them and then under the Options tab you will see Volume Adjustment.
There is a bit more info in this Knowledge Base article that may be of help, such as supported file types.
If that isn't sufficient you'll need a 3rd party plugin to give you more control, but I'm not that familiar with your choices there. Macupdate.com would be a good place to search.