audio driver not recognized in Vbox 2.1.2 with Vista 64 VM

When I try to use my Vista 64 bits Pro VM in VirtualBox 2.1.2, no audio peripheral is found whereas sound is working correctly under the opensuse 11.1, kde 4.2 host and ICH AC97 or Soundblaster is configured in the Vbox settings (with ALSA host driver).
Apart from that the VM is working correctly

Any idea ?