I have noticed there is a difference between Oblivion intro movie music and in-game menu music so I've wanted to find a longer version of the "Reign of the Septims" song. I believe this is similar to the background music during the intro.
All credits goes to Bethesda.

INSTALLATION

Simply put the tes4title.mp3 into "Data\Music\Special" folder and replace the original, or you can put the song into whatever music directory you would like (explore, public, dungeon, battle)

Note that you can change the menu music volume in your Oblivion.ini line fMainMenuMusicVolume=