If memory serves me right, Vuo deals only with 48 kHz audio. If that is still the case, the 32 kHz limit of the C922 could explain the problem.
Maybe if the audio goes through a virtual audio interface like Blackhole or Loopback, the OS will be able to resample the audio stream to 48 kHz before sending it to Vuo.
Even barebones, the gritty slowdown happens... Maybe a bug?
The bell file is "dual mono": mono signal in a "stereo" file.
The chime file (which to my ears "should" be played an octave higher) is full of distortions: maybe this is what you're hearing?
How many channels in your files? Because "Mixed Samples - A single audio buffer consisting of all channels added together." So... Two stereo files would be four channels added together?
Play it safe and adjust the loudness to 1/N. In this case, 0.5 (6 dB softer).
0.84 is 1.5 dB softer: more often than not, this is insufficient when adding two audio signals together. It should be at least 3 dB softer (0.707).