I use an external DAC on a linux system. This used to work fine on an IBM Thinkpad. On the nuc I get frequent crackles so the device is practically unusable for audio playback. Tried different distros with different kernel versions (Debian stable and testing, Fedora 20) with the same result.
I am sorry you are having this problem but let me guide you.
It is important to say that the supported Operating System for our NUCs are windows versions so there are no drivers we can provide in this case. A basic step would be to test a different DAC if possible active.
You can contact the Linux Developer Forums or hopefully a user with Linux experience can help you with some steps to get it fixed.