To my knowledge the only stand alone speaker set that was designed for consoles had modified inputs. (They weren't like PC, they were like 'normal' inputs.) These were sold at places like EB Games / GameStop / Frys, but I haven't seen this set in years.
It does appear that you could run an input into the X-Fi Fatal1ty
, but that's going to run you ~$180.
It looks like you could also go with the very nice Logitech Z-5500
5.1 speaker set, which has an optical input. But, that's gonna run you ~$300. Though several reviews have said these are the absolute top of the line for 5.1 systems that near home-theater system quality. (I don't actually see where the input is though.) This may be the better choice if you want really nice sound. No offense, but those speakers you have are decent for PC gaming, but I don't know how good they would sound when running movies or games from an external source.
Do you not have the option to hook up the Xbox in the living room to a real sound system?