i'll try to explane

i hope you'll understand my english.
first of all i think it wil be much more easy if you don't use the pc. i'll explane you how.
first have a look at the cable of your speakers, i think you got 3 posible cables
cable 1: jack to jack
if it is one like this. i recoment you buy an other one a jack to rca (see below) so you can put the red&white in the mixer straight into the speakers
cable 2 or 3 (jack to rca or rca to rca)

or

if you got one of these its easy just put one red&white in the mixer and one in the speakers.