Adjusting Volumes

I have 5 songs which I did fade ins and outs as a mix, on 1 stereo track, and some songs are louder or lower than the others. What is the best tool to use to even out the volumes?
One song in particular is quite a bit louder than the others…anyway to highlight that segment to lower the volume, or will a tool do it for all song levels?

Probably the Envelope Tool Audacity Manual