sudeshnas
It's not only the HD Audio, but other settings too (e.g. virtual floppy) which do not show up in the HTML5 VM settings dialog. However, some of the settings show up on the VM's General pane. I got feedback that the developers have been made aware of this, so I assume this will be fixed in an future version/patch.
In the meantime one can to either use the flash client, or the VMRC to edit such settings.
André