Have you tried to Mute the sound in BSplayer?
You can either remove the audio in optical cable, or select another source for "Audio in" in Capture settings window (if you choose Microfone when you have audio in the Line-in input you will not have sound..)
If the problem is in the captured file size, then choose the lowest quality for audio (it may be 11025Hz @ 8bit), and if your PC is fast enough enable also a sort of compression for the audio.
After the movie as been recorded, you can use an external program (VirtualDub Mod for example) to delete the mute audio track
