Can't remember if the 560s are 4.1 or 5.1, but you'd hook them up:
Gameport-------Rear-----Front-----xxxx-----xxxx-----Digital
/======/---------o----------o----------o--------o----------*
On a 5.1 soundcard.
Se the vlume lower than max. Do about 75-85%, as Ruharc mentioned. This keeps the line from being 'oversaturated' in sound and creating 'farts' in the speakers. Especially important for high-bass applications.
No, there are 2 volume controls. Srry. Get used to it.