Well...that's a pretty big question since we don't know what settings you changed or what audio interface you're using.
However, the first place I'd have a look is on the Edit/Preferences/Audio Hardware menu. There you'll see a menu for "Device Class". If you have an external interface, this should likely be ASIO. if you're using the built in sound card, it'll be MME. This applies to Windows--if you're Mac user, I'm not sure what settings there should be.
Anyway, in Windows anyway, the Device Type selection will give you a drop down menu (or two for MME) where you can specify what sound device to use. Hopefully this will sort you out.
However, (Again in Windows) if you can't find your Audio device in Windows, the next step would be to go to the Windows Audio control panel and check that you device is shown there and, if so, make it the default. If you don't see it, right click on the white are of the control panel and make sure the options to show Disabled and Disconnected devices are clicked. Hopefully the right device will show up and you can go back to the Audition Audio Devices panel and set it up.
Sorry about my lack of Mac knowledge.
if the above doesn't help, probably best you come back and give us details of the sound card/audio interface you're using and also any memories you have of what settings you changed.
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