I'm assuming that your Home Theater is capable of doing at least Dolby Digital Pro Logic, so, when you have your Receiver set to Aux, look for a button on either your remote or receiver that looks like this: http://images.techtree.com/ttimages/story/74647_dolby.JPG (Not all the words, just the two D's...)
If that doesn't work, you might have to get an external sound card and use some kind of digital source like Coaxial or Optical...
If you can give me the model of your Surround Sound system, I might be able to give you some more insight into your problem.