Check your drivers... I find it quite odd that only your front ones work as it's usally the opposite with other people! So...I'm expecting you might have some sort of 5.1 or something setup any perhaps the computer just isn't playing the correct audio though the ports(ie, nothing) I have a similar problem in games when I'm in 5.1(the only way I can get my sub working) because I don't have audio in some games...
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Desktop * Athlon X2 4200 | 2G RAM | 160G HD | 7600GT
Laptop1 * C2D T5550 | 3G RAM | 120G HD | Intel x3100
Laptop2 * C2D T5250 | 2G RAM | 160G HD | Intel x3100
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