Can't you change track with DivX 6 even if you click (or right-click) on the DivX traybar icon? (I don't know if that is the way it's meant to work, I don't use v6)
But if you can't see the multiple tracks with BSplayer (and different codecs) but you see them in VLC, then I think that your problem is the splitter..
In the Filters window (the one pointed to you by BSPeter) you can also find splitter info (usually under Source item)
Try to re-register quartz.dll:
START -> Run... -> regsvr32 quartz.dll |