You don't need ESX/VI3 to emulate a sound card for RDP to remote sound.
In my VDI setup:
I have an ESX server with no sound card emulated and I have RDP sound remoted to the clients from an XP VM hosted on the ESX server. All I did was turn sound remoting "on" in the RDP session.
It is handled on the client side and will be specific to the vendors client / open source client you are using. If the RDP client provider ha not implemented the capability it will not work.
Same goes for Smart Card passthru, client drive mapping, printer redirection etc.
