The problem with onboard audio is that it doesn't always play well with dedicated sound cards. I would recommend that you turn off your onboard sound device in the bios and get some hardware that connects to the soundblaster card to drive both the speakers and your headphones. Many PC speaker systems, like the logitech X-240 -link below #1-, have a little satellite device that sits on your desk. It usually has both volume control for the speakers and a headphone jack.
If you really want to use your current speaker system, you can get a little "Speaker/Headset Switching Hub" -link below #2-. These devices plug directly into your sound card and then run the sound for your speakers. The only down side is that most of them only support 2.1 sound, not surround sound.
A third option would be to get a USB headset. The downside of this option is that you have to change sound hardware settings when you want to use your headphones. I have friends who use this option and it seems to work well.
If you want to use your front panel audio ports, I would look and see if your soundblaster card has audio headers. Look for "Intel HD Audio Compatible Front Panel Header" in the specs. I don't think the cheaper ones have it and the more expensive ones sometimes come with something you put in the drive bay. I think all the X-Fi cards have it.
Keep in mind that you could have some faulty hardware somewhere if you only get sound out of one channel. I would try testing out each port with a known good set of headphones and see what kind of audio signal you get. If you have surround sound, you may be plugging into a rear channel by accident.
Hope that helps
Links (New users can't post multiple links)
#1: Go to logitech's webpage and go to the speaker+audio section. If you look under speaker subheading, the X-240's are at the bottom.
#2: Go to amazon and search for "Manhattan-Speaker-Headset-Switching-Hub" it's the only result at $9.99 .
1have you tried fixing the root problem -- that the SBL rear output is only sending left-side audio? have you checked whether other rear ports (if any) are set to send right-side audio? i wonder if you don't have a setting somewhere that is set to one-channel-per-port. – quack quixote – 2009-10-04T16:40:02.937