No output to rear speakers

Ok so I have rear output to my speakers as long as I'm using my computer but I have my tv connected to line in on my onboard sound card (headphone jack adapter from red/white) I understand its only going to be in stereo but is there any way to duplicate the sound in the front speakers to the back? If I use say vlc to stream line in it plays all 4 but there's about 1/2 second lag time between audio/video. Also I was looking at Creative - PCI Express Sound Blaster X-Fi Titanium Fatal1ty Professional Series Sound Card because it has optical in but will that clear up the lag time?

That doesnt stream audio from line in port (that i noticed anyway) and when i use vlc or winamp audio works fine through all 5 speakers its only when i use my tv. it sends audio to my line in and the desktop plays it in real time w/o needing a program but it only plays front 2 speakers and i want it to play in back as well as front.