Altec Lansing 5.1 speaker proble

Hello, I have Altec Lansing VS3251 5.1, now the problem is when the speakers are on 5.1in rear speakers are activated but when i push Ster/Surround only front RL give the sound, i use windows 7 and do not have sound card, which basically means, 5.1in + Ster/Surround will only activate front RL speakers not the rear 3 speakers.