To get alternative music on the character selection screen, namely 'I Want To Take You For a Ride' remixes, hold L2, R2 or both together when selecting the character selection screen. Each combination will get you a different one of the three songs.
If you do not have a Marvel vs. Capcom 3 save file to get Galactus the easy way, you will need to unlock all 48 character endings first. You then can hover over Arcade Mode, hold Select and L1, and press A to select the mode. You will then be able to play it as Galactus.