So far SX (SL) only provides a "limited" support for USB audio interfaces that do not come with a specific ASIO or CoreAudio driver. Due to the special character of the built-in USB audio support in OS X these devices cannot be used in full duplex mode with SX. This means there is only an input OR an output available. OS X virtually addresses the different in- and outputs of a USB audio interface as single devices. Unfortunately SX cannot handle that yet.

Possible Solution: Check if a dedicated OS X driver is provided by the manufacturer of your interface. It is planned to implement the "driverless" support of USB audio devices in a future SX update.
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