You can unlock a Cheat called 'Disco Mode!' then enable it in the secrets menu! Mike changes his clothes for the occasion, along with flashy colours and some groovy tunes!!
Gardelle County Jail (Helicopter Area) - Go to Dr. Death's Schism Gate and go to where you fight the helicopter and destroy it. Then, go to the tunnel to the right of the entrance. The "Secret Activated" message will appear.
Go to the Cathedral Of Pain and drop down the shaft into the Marcher lava. Go on the top of the spinning blades, then find your way to another shaft that goes down. Drop down and the "Secret Activated" message will appear as you fall.
Not in the N64 version.
L'Eclipser is required to get through the saloon area in Salvage Yard as Mikey can't walk on the fire long enough to survive. At the end of the level, you'll come to Marco Cruz's disco sanctuary. Once you've taken him out, you'll notice his DJ board on the stage. Head on over to it and the "Secret Activated" message will appear.