With a USB DAC on an IdPi, you'll need to configure audio device from Tools -> Options -> Audio. However, sysdefault is probably not the right option. I'm not sure how the audio device list is being populated because of your DAC, but you need to select one that indicates the DAC. You may need to try different options, but you should start with one that begins with "hw:"
You can also use option 10 from the main menu to show audio device information. Make sure your DAC is showing up as one of the audio devices. If it is, you can see how your DAC is displayed, such as the card ID and the device number and ID.