unless your amp has a built in decoder you wont get surround sound out of the Xbox 360.
basically your amp needs to be able to decode either dolby digital from an optical toslink connection or Dolby pro logic from an analog connection.
The Xbox 360 was not designed to work with PC speakers as PC speaker rely on the computer to perform the decoding and accept line level (pre-amp) analog data rather then encoded data. This is done to save money since it's easy for a PC to do that sort of thing. Home theater equipment (like game consoles) is not designed that way and almost always use an encoded audio signal for audio (untill you get up into the systems worth 10s of 1000s of $$)
To use an Xbox 360 (or any console) with PC surround sound speakers you'll need a stand alone decoder to interpret the surround sound signal.
The cheapest stand alone decoder on the market (that I know of) is the Creative DDTS-100 which retails for $150.