Sorry for the conspiratorial wall of text, but I've been doing some testing for the last hour or so and have come to a weird (and possibly completely wrong) conclusion: I think the new Series X controller causes wireless audio interference with the console. Ever since my Arctis 9x showed up and I've been getting the same occasionally extremely choppy audio, I've tried to narrow down what it could be and the only consistent variable seems to be the controller that came with the Series X.
When using the Series X controller, if I let it idle/don't touch any inputs for 10 or so seconds, headset audio gets extremely choppy/stuttery. This occurs with both my wired Turtle Beach headset and my new Arctis 9x wireless headset. My (headset) distance from either the console or controller does not seem to influence this (based on the limited spacing to test in my apartment anyway.) Moving the analog sticks or pressing a button instantly causes the audio to clear up and become normal until, again, no controller input occurs for 10 or so seconds.
I've further narrowed it to the Series X controller because using my other controller (the Bluetooth Xbox One controllers that came with my One X) yields no audio interference as described when using the Series X controller. Idling, no button inputs, etc - no audio choppiness. Having both controllers on and synced to the console - no audio choppiness. Turning off the One X controller with the Series X controller remaining on - audio choppiness while idling comes right back.
Am I onto something here or just out of my mind?
Edit - Was about to post this, but I've been sitting here trying different combinations of which controller is on/off, which one is on first, etc while writing this post, and just found an interesting wrinkle: if the Series X controller is on first, my One X controller can't/won't sync to the console at all. Trying to power it on causes the Xbox button to fade up, blink a couple times, then turn off. Turn the Series X controller off first? One X controller turns on and syncs just fine. Haha, what's going on here?