As far as I understand, it is essential that the xHCI USB3 controller is enabled (in the guest settings) AND that the Guest OS is Win7 or above, BUT the fulfilment of those two requirements is not enough to guarantee that the audio soundcard will work as expected. Many "strange" things may still happen: for example, some USB cards seem to be regularly "captured" by the guest (complete with regular driver installation) but then there is no actual audio output, or the audio output is noisy or faulty in some way.
According to what has been reported inside various forum threads, sometimes the problem gets solved, more or less... permanently, by certain "classic" maneuvers (re-installations, reboots, reconnections etc.), while in other cases there's simply nothing to do.
I also found a quite clarifying technical explanation written by the user "michaln" back in july 2018:
"Some USB audio interfaces are reported to work acceptably, which is no guarantee that another interface will work -- it all depends on the details of how the USB device and its drivers are implemented."
So, in the end, some cards will work without problems (although obviously somehow penalized in performance), while others won't at all, with many possible intermediate situations. Which is no good news to those who have to choose and buy a card, since quite few manufacturers provide indications about compatibility with virtual machines.
Coming (finally!...) to the main reason for this post of mine, here is my question to other forum members:
Maybe we could share such experiences and create a kind of (upgradable) list which could be useful to the VBox community.