I’m having the exact same audio issues with SNES (Both a launch model and a 1chip console, neither with the dejitter mod however) on the Avermedia Live Gamer 4K too. It’s been driving me crazy for about a few months now, and I never considered that the 2004 Windows update was the culprit. I rolled back drivers and tried every piece of recording software I could to no avail. It’s a shame, the GC573 was perfect up until recently. The most recent 2020.08.11 firmware hasn’t changed anything, too. Everything else I’ve thrown at it (Genesis, TurboGrafx-16, PS1, PS2, Saturn, N64, Dreamcast, Gamecube, Xbox, and Wii) still works perfectly on every mode, it’s just SNES that has the audio issue. I even still get clipping at 2x, but it’s very infrequent compared to to 3x and especially 4x and 5x where it’s once every few seconds compared to 2x clipping for me once every few minutes.
I’m really curious if NES has the same issue as well, but I don’t have an RGB modded unit to test. Either way I hope the issue gets addressed by Avermedia at some point, but I’m doubtful considering how oddly specific it is.