OK, sometimes I forget that some folks arent audio geeks like me, so I wont be super clear. please forgive.
we might have some confusion on size of connectors...1/8", mini, and 2.5mm all refer to the same size- the green jack on the soundblaster card, and plug from the speakers that goes in that card. most consumer headphones also use this.
RCA is the standard red and white on the back of the stereo, for interconnecting components.
assuming the headphone to RCA goes from the 1 plug (mini) to 2 RCA (red and white), that is a splitter.
heres the Y cable (in this case its solid plastic, same function) 1 male mini, to 2 female mini.http://www.radioshac...a..._id=274-313
(if you look at page 1 on the radio shack link, you will see the ridiculous prices for monster cable versions of these. rip off artists
...but I digress)
plug this into the green jack on the soundblaster. plug the mini connector for the comp speakers into one jack of the Y.
plug the headphone extension in the other jack of the Y, then the splitter cable into that, and finally the RCA end into the stereo.
the 2 soundblaster option wouldnt work, since only one could be selected to play at a time. for some reason, an onboard card can work with an aftermarket card- but again, they ae not synched up, so the sound comes from one slightly after the other, which is no good.
hope this clears it all up. let me know how it goes.