Previously, the msiounit filter for IOS had created the au_card object to host an apple AudioUnit and some extra context informations, and each audio device (represented by MSSndCard) would have its corresponding au_card.
However, MSSndCards should only be used to represent a device and not save user data, and only a single au_card is really needed for the whole app.
Therefore, we replaced it by a new singleton interface, to make it clear that there will be one and only one AudioUnit created and configured at any given moment.