I'm doing research on using the S20+ to record video with an external audio source, so I don't have any hardware to work with at this point. But from what I have read above and how phone stuff normally works, I have a few observations to toss out there.
The USB-C to headphone cable is likely just a simple headphone adaptor for TRRS type headphones, and the headphone with and built-in mic and control buttons works a lot like the old 3.5mm TRRS jack.
The USB-C to DAC cable has a Digital to Analog Converter (DAC) chip, so that adaptor is actively converting and analog microphone signal to a digital signal that is passed to the phone. If you try to send a much higher line-level audio signal into the DAC which is likely setup only for the lower microphone audio levels, the DAC will be overloaded and create stuff like screaming and unintelligible noise. You would likely need to reduce/pad the external line input to make it compatible with the expected DAC microphone input levels. Unless the DAC is supposed to be able to handle both audio types correctly?
Most phone compatible TRRS headphones with control buttons uses special configurations of things like resistors to signal to the phone how to deal with button presses in addition to audio input and output. Because different phone manufacturers use different signaling configurations, this is why you need to find select a TRRS headstone/mic product that is compatible with your specific phone model. With these new DAC adaptor cables, the lack of these switch signaling resistors, or the wrong kind of headphone/mic cable configuration, may be what is confusing the DAC cable. It may be reading a "constantly pressed button" which in fact is not really there, and then sending the phone an incorrect signal to do what that button wants such as starting to play music when not in a recording mode. If the adaptor manufacturer were to provide better information on their products, like how to properly wire an external cable to work with the DAC adaptor, this may be the answer to the constantly playing music issue. But I rarely see such technical data provided openly to the public, especially when the parts are new and rushed to market. Eventually someone may figure out what is needed on the cable end to fix this problem, but why the need for someone to figure it out instead of just providing the details up front?
Has anyone seen a Samsung compatible audio adaptor that also supports external cable phone charging as well as the audio interface AT THE SAME TIME? I have run into the problem in the past where adaptors work great for what they are supposed to be doing, but also block charging. So instead of keeps the phone fully charged while shooting video, the video shoot will only last for as long as the external battery can keep up, then you end up with a dead phone.
Just reading about issues like new audio adaptor cables that seem to crop up from a lack of basic testing when they make changes like removing the old 3.5mm TRRS jack doe an adaptor cable setup is what concerns me with using the latest, greatest, and most expensive equipment that fails some basic functionality.
