Which Behringer DAC are you using? There's probably a switch/setting that needs to be adjusted on the DAC. I'm using a very basic Behringer mixer, and if I don't get certain buttons set correctly, I get behavior like that you're describing. It would also help to know exactly how you have the hardware connected.
BTW I read the description of iShowU Audio Capture a quick read. I think that extension allows recording Mac's internal sounds, such as system beeps. I don't think that's what you are trying to do.
Here's my current setup for recording from my stereo to the computer. Something similar should work for you.
1. The mixer I use is connected to the Mac via USB.
2. The left and right channels from whatever stereo output I am using are connected by cable to the left and right side of an input channel of my Behringer.
3. In the Mac Sound preference, pane input is set to USB Audio Codec. Output is set to whatever sound device I want to use to hear the recording. It's usually set to the Behringer because I have headphones connected there, but I've also used the Mac headphone port or a Bluetooth speaker/headset.
4. In Audacity, go to Preferences>Devices and set the import device to USB Audio Codec.
Now try a recording and see what happens.