Maybe it's a codec/splitter problem, or it depends on the rendering type you choose in BSplayer preferences (it works perfectly with default renderer, but recognizes rmvb files as audio only if overlay mixer renderer is selected).
Does this happen with every rmvb file?
Try to download some samples from
this page and see if they work or not.
I've not tested all of them, but I can assure you that
ScoobyDoo2 trailer (1Mbps) works perfectly with BSplayer and Gabest's Decoders and Splitter
You can also try to use the BS.Player codec manager available in latest release (ATM v2.30.970), just install v2.30 or greater and play your rmvb file, if it plays normally (without asking you for a codec update), then go to your BSplayer installation dir and run
bspcodecdl.exe executable.
Then select RealMedia Splitter and click on "Install selected" button