Core Audio

First, go to System Preferences>Sound, click on the Input button and see what options you have, then do the same with the Output button. (And, write down the list of options for your next post, if this doesn't fix it.)

Then, go to your Utilities folder (shift-apple U in the finder), and click on the Audio MIDI Setup app.

In this, click on Window/Audio Devices. What do you see here? Note that there are icons in the list (a Finder, a microphone, a speaker) showing what's currently being used. To change the devices, click the sprocket at the bottom ("Use this device..."). Try setting everything to internal, and take note of any other options that Premiere may have set up.

You may also try clicking on the output device, clicking on the sprocket and selecting Configure speakers...

If something here doesn't look like it's causing your problem, at least you'll be able to change things in Core Audio according to what Adobe wants.

Good for you for posting that great info! Sorry I haven't been back to answer you.

If there's feedback happening, just set the volume of your USB mic (AKS370) to zero for the time being.

To listen through speakers, I believe that in Premiere, select "Built-in Output", and to switch to headphones, select "Built-in Line Output".

That's the basic checkup, if that works, try this, keeping in mind that I don't know Premiere:

The Premiere Pro 6.0 button (with the blue cross) in the Audio Devices window of Audio MIDI Setup is a combination I/O set up called an aggregate device. I am not sure, but that's probably what Premiere uses for its System Default Input/Output setting (if it's not using the Apple settings). Click on that, and you should see a window that allows you to change your outputs so it will use the Mac internal speakers (Built-in Output) instead of headphones (Built-in Line Output). At the same time, set the input to "Line Input" or you'll get feedback when your mic level is turned up. Then press the "gear" at the bottom, and check Use this device for sound input and Use this device for sound output. The microphone and speaker icons should both appear next to your Premiere aggregate device.

If you need to use your microphone for narration or something, I would set up two aggregate devices: one that has the microphone for input and headphones for output, and another that uses line input for input and speakers for the output. Switch between them in AMS and you switch both together.

This is not a definitive answer, but since no other Adobe users have posted something more authoritative, this is what works in other audio programs. Good luck.

P.S. Of course, you can turn off the speakers simply by plugging in headphones, but you shouldn't do the same with a USB microphone...