Hi,
I have same problem, same motherboard (I assume you use onboard sound like me), and different sound system(Tevion 5.1). It seems that movies with AC3 encoded sound work fine, but those encoded with mpeg layer3 or any other stereo codec output sound only though the front speakers (i think this is because the sound card on the motherboard doesn't know how to filter and output the stereo sound though all channels). I had same problem in winamp, but luckly in winamp i found a very very nice plugin called Dee3 (
http://dee3.tk/) which does the job of outputing through all 6 channels right (filtering bass and all).
Now I am asking.... (as I saw there was an option to load DSP Winamp plugins in Bsplayer), is there any way to load this Output Plugin from winamp into BSplayer to output the 2 channel encoded sound from movies through all 6 speaker and filtered ? Please help :)