Thank you Aaron for your reply.
In fact a lot has happened since my last post here. You can read the whole story here if you have some interest in using ESXi in a workstation setup: https://www.patpro.net/blog/index.php/tag/virtualisation/
it starts at the bottom of the page.
Unfortunately, Apple hardware does not allow proper sound device passthrough into a VM. I was able quite easily to setup passthrough for the sound controler, but a Mac OS X VM won't recognize it, and a Windows VM can use it but sound is awfully deteriorated, and the driver appears to commit suicide after only few seconds of playback. More importantly, the VM can't read a youtube video properly if sound passthrough is enabled, when it can read flawlessly the same 1080p HD video if sound device is not added to the VM. Same problem if I try to use a logitec USB headset for sound playback. May be I need a dedicated PCI sound card... I won't try.
I would already need a dedicated PCI graphics card for each VM and a dedicated PCI USB card for each VM, there would be no room for a dedicated sound card.
I know ESXi is not built to support proper sound playback via passthrough device, its written somewhere in the documentation. I see many youtube video about gamers playing in Xen VM with sound support. May be it's the way to go, but I need to buy proper hardware (supermicro's Whisper Quiet Workstation for example), and more importantly, I'll have to ditch Mac OS X. Not an easy thing to do!
(edit: typo)