Why doesn't unity audio work with 4 channels

Question

I'm trying to figure out how to do multi channel audio in unity, and it looks like something isn't working correctly.

Here is my setup:

• Blank unity project (unity 2020.3.0f1)
• Set project settings > audio > default speaker mode to "Quad"
• Create a 4-channel audio clip with audacity (1 "click" on each channel, one at a time)
• Import the clip into unity with default settings

I don't have an external sound card (yet) so I'm using VB-Cable as a virtual 4-channel output

VB-Cable provides a Control panel where I can see the levels for each channel. Playing back the source file with quicktime, I can see the channel 1,2,3 and 4 levels moving as I expect.
However, playing back the imported audio clip in unity's asset preview merges channel 1 with 3, and 2 with 4. The same thing happens when playing the audio in-game.

Another setup I've tried is the same as above, but setting up my computer to use an "aggregate audio device":

• 2 outputs from my built-in output
• 2 outputs from VB-Cable

(I also changed the VB-Cable settings to make it a 2-channel output, so that when combined with the built-in output, I get a 4-channel output device)

With this setup, playing back the clip in Quicktime or in the unity asset preview works as expected (I hear the channels 1 and 2 through the Built-in output and "see" the channels 3 and 4 on the VB-Cable control panel). Playing the clip in-game however, only the channels 1,2 and 3 seem to work. Nothing happens on channel 4 (Using a unity audio mixer, I can see channels 1,2 and 3 "lighting up" but 4 stays silent)

I know changing audio config with unity opened is not guaranteed to work, so for every test I do, I quit and restart Unity.

I hope I've explained the issue properly. If not, here is a zip file containing the test unity project (including the 4 channel audio track), and screen recording of different tests I've made.

System:

• MacBook Pro (Retina, 13-inch, Late 2013)
• MacOS 11.6.2
• 16GB RAM
• 2,8 GHz Dual-Core Intel Core i7