Here's a recording of what my audio output sounds like. I am using a USB sound card identified as C-Media Electronics, Inc. CM106 Like Sound Device which is outputed over optical SPDIF to a Denon 5.1 receiver. In Windows, I have no issues with the audio. Under Ubuntu 16.04 there's a constant clicking/popping which is louder as the sound is louder.

I visited other posts and followed a few suggestions like installing pulse audio, changing mixer settings, and other attempts but it still continues. It doesn't happen over HDMI or analog.