Here's a more detailed response that i had posted elsewhere:
My computer has Realtek's HD Audio Manager installed, and recently, I bought a subwoofer to add to my speaker system. Previously, I had been using a quadraphonic system. Now, I plugged in my new sub into my jack on the back. The jack, according to Realtek, is a center/subwoofer jack. So, after plugging it in and testing that it does work, I went to the manager, and told it that I had a 5.1 system. On the side, it gives me boxes corresponding to the speakers that I have. So I unchecked the box that says center speaker, indicating that I do not have a center speaker. At this point, I've said that I have a subwoofer and 2 rear speakers, but no center (no front speakers is not an option). Apparently, since the jack on the back is a center/subwoofer jack, when I say that I don't have a center, Realtek stops sending out a subwoofer signal. I've tried setting the crossover on the sub to as high as it can go, and let Realtek use it as a center, but it sounds AWFUL. However, if I plug my iPod (using a headphone jack to headphone jack cable) into the line in input on my sound card, I can use my sub without the center speaker box checked. Please help, I really want to to be able to use my sub with games and my music without having to plug my iPod in. I've looked on the internet, and I haven't seen any posts like this, and no solutions. I have the most recent Realtek HD Audio Manager, and my system is a Intel Core i7-920, stock clocks and Vista 64-bit. I do not use an external sound card.
Looking back, I'm not sure why i didn't post more. Anyways, I've tried using it as other speakers (to Realtek), but it still doesn't sound good. Any help would be appreciated.