Reason works with a Core Compliant audio interface or your built-in audio hardware. If you want to use yet another audio card – like a USB mic or a USB guitar cord – you need to create an aggregated audio driver that combines the input from your microphone with the outputs from your audio interface. Here's how to set up an aggregated device using (for example) your USB microphone as an input, and the headphone out on your computer as an output: Mac OS X 10.6 Snow Leopard and later
Open "Audio MIDI Setup" in the "Utilities" folder in "Applications".
Make sure the "Audio Devices" window is open.
In the lower left corner, click the [plus] button and create a new aggregated device. IMPORTANT: Also MANUALLY RENAME your new aggregated device!
Tick the "Use" check boxes for each of the Audio Devices (inputs/USB mic and outputs) you want to use.
In Reason, go to Preferences/Audio and select your new aggregated driver.