As someone which designed such audio UIs with knobs, it's not because skeuomorphic was the target, it's because there is no better way to fit 50 controls in a small space while being able to see them all at the same time.
There's plenty of UI that presents drop-down (or pop-up) sliders.
I will say, though, that a circle whose perimeter is partially filled in is a space-efficient way to depict the current value vs. the total range available.
Yeah, I've seen this resentment about audio UI a lot here on HN, some of them sound more ignorant than others, but I'm yet to hear a convincing argument against knobs. People should probably check out FabFilter products, best UI in pro audio :)
I will say, though, that a circle whose perimeter is partially filled in is a space-efficient way to depict the current value vs. the total range available.