Sound indeed works on OS X 10.6 (Server) in Fusion, with the VMsvga2 AC97 driver, available from the link:
EnsoniqAudioPCI_v1.0.3_Common_Installer.pkg
... and this package must be installed inside the virtual machine, i.e., inside the guest (not in the host!); before using this driver, one must create a new sound device in the VM's settings, and then edit the .vmx, changing the line:
sound.virtualDev = "hdaudio"
... into:
sound.virtualDev = "es1371"
... and sound will then work, albeit certainly not with a very high quality.