Ooh, fun question. :D

There are probably more and better music choices, but these are some songs that remind me of certain characters (or vice versa):

Java: Andy Pickford - Cloudwatching

High Priest: Andy Pickford - Night of the Long Knives

Lucideus: This Mazedude remix of Myst III - Wheels of Wonder:
http://ocremix.org/remix/OCR01749/

Joe: Harold Faltermeyer - Axel F

Kiku: Jim Carrey - Cuban Pete

Some guys who haven't been introduced in the new version, and must remain nameless:
- Voltaire - When You're Evil
- Alice Cooper - Poison
- Stray Cats - Puttin' on the Ritz
- Shandi - He's a Dream

I'd love to hear other theme song suggestions in the forum. ;)