I have been using Virtual DJ on my machine. The setup is where I want audio to play over the headphones and use a USB sound card to play audio into my headphones so I can listen to music before I play it for everybody.

![enter image description here](https://i.stack.imgur.com/3KOQs.jpg)

However, the last time I tried, I got an error message saying "Error in the sound driver of Core Audio. Your sound card is probably not connected, or the settings are incorrect".

![enter image description here](https://i.stack.imgur.com/jyORb.png)

If I use the headphone by themselves, the USB sound card by itself, or my speakers by themselves, they all work individually.

How do I go about figuring out what's wrong here?