Use Ventrilo, it lets you specify where it outputs its sound to as long as the headset is detected as a separate audio device. If you have Vista then most likely your BT headset will be detected as a unique playback device and if you use Ventrilo, it has options to specify the output device. Also, check your drivers, my onboard Realtek drivers have an option that states "Make front and rear output devices playback two different audio streams simultaneously" but that only specifies for things that are connected to the sound card.
Summary:
-You have speakers connected to motherboard - Vista sees this as "Speakers" (usually) and sets it as your default audio device. Every sounds comes out of those.
-Your BT headset is detected as a different audio playback device.
-Ventrilo has an option that allows you to select the output device. Thus making only the voices come through your headset.