There are two places to check in OS Tiger, Leopard or Snow Leopard. One is your "Sound" preferences, which has an output selector (make sure your usb device is connected). The second is in the MIDI preferences, which for some reason is found in the "Utilities" folder. It also has an output setting that must be set as well (to the same output as you chose in the "Sound" preferences). In addition, the MIDI control panel allows you to set the sample rate and bit depth. Good luck. Cordially,
