Equal volume across a long track of backing music

[Trebor]

I guess my key question is still - do any of these plugins even out the loudness of sections several minutes apart?

Compression, AGC, & LevelSpeech will all reduce the dynamic-range.
Usually downward compression: sounds above a threshold are turned down.

When I put that track behind the spoken audio track though, the difficulty arises.
No matter what I set the gain to, either some sections of the music are too quiet, or too loud.

Another option is to AutoDuck the music when the person speaks, so they can be understood.
(IMO Steve’s Dynamic Mirror plugin is easier to use than Audacity’s Native AutoDuck).