whoa nice hl2 pic behind!! hehe
1)the existing s/card speaker out to line channel 1 of your speaker, then to your powered speakers. but you will not have control over your audio on how you want to blend two tracks in. the mixer acts as an aux control parameter & a preamp for your audio signal thats all.

2)as what chanfreak says. get an aux s/card with multiple outputs. 2 min. you can have more outputs provided you have a multi channel speaker. assuming you have a 2 channel output s/card, output 1 of s/c will go into line input 1 of mixer and output 2 of s/c will go into line input 2 of mixer. then from a dj software, sequencing software (which enables you to sream audio via multiple tracks) adjust track 1/ turntable 1, & configure it to stream out only via s/c output 1. and track2/tt 2 stream via output 2 of s/c.

3)other methods? i think of sommore.

i just heard about the DMC m'sia ... really interested to see m'sian standard...i hope the big boys will be competing ..

i was at infekted event today .... missed xtothed's set ... i was late... stucked in traffic jam and also got lost on the way..
overall .. i was there to support and also stretch my ear drums .. lol ... loud musics with lots of kids ... so yea...

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Added on August 23, 2008, 9:05 pm

keep your existing built-in sound card as .... output 1 ....
buy another PCI/USB sound card... as your 2nd output ...(this maybe RM 20 ? .. i don't know about the pricing)
hook it all up into your mixer input ... and your mixer main output to your speakers..

this method, you will need your mixer to be turn on each time u need to output audio tru your speakers..
so even if you are not mixing, and just watching youtube or something .. u need to turn on your mixer & speakers...