Each song can have it's own Volume such that, when the song is played, it automatically sets the volume. To show the Volume meta data, Context Click on the sort column headers, then pick "Volume" from the list. Now you can see the volume associated with each song.
To have a song remember it's volume, edit the Volume for that song and set it to 1, or, start to play the song then pick "Controls->Remember Song Volume". Then, when the song is playing, any adjustment you make to the volume knob will also cause the song to remember that volume.
To have a song "forget" it's volume, edit it's Volume and set it to zero, or, play the song, then pick "Controls->Remember Song Volume" to toggle it off.