Use something like this circuit as I described above :-
Simple Line MixerYou'll need two one for the left outputs and one for the right outputs.
Most cheap mixers only use resistors. There will be slight feedback but not enough to make any significant difference.
If you really want to try something more complicated you can try something like this :-
Stereo MixerThe problem is this circuit needs a split rail supply. + and - 9V because the opamp has to drive the signal positive and negative just like the audio input coming from the xbox or cd player. The xbox doesn't have a -ve supply voltage from the power supply so you would need a converter. Stick with the easy option of the first circuit.