Extra audio device. This class allows application to have multiple sound device instances active concurrently.
Application may also use this class to improve media clock. Normally media clock is driven by sound device in master port, but unfortunately some sound devices may produce jittery clock. To improve media clock, application can install Null Sound Device (i.e: using AudDevManager::setNullDev()), which will act as a master port, and install the sound device as extra sound device.
Note that extra sound device will not have auto-close upon idle feature. Also note that the extra sound device only supports mono channel.