Update from issues discussed here:
Today I had time to tear things apart and look into this some more.
Here's what I attempted:
I replaced the inner USB hub in the console with the one from my jump seat. They are identical, and the swap didn't change anything; same issues as before: no USB media or AUX playback.
I disconnected the inner USB hub from the front USB hub and connected it directly to the HMI. USB media then worked on the inner hub. At first AUX audio still didn't work but then I tried a different audio cable and finally AUX started playing for the first time. I noticed that AUX does use the USB connection even though there is separate wiring going to the hub for it, if you disconnect the USB connection it stops playing. Also, AUX now always appears as an option when cycling through "Media" inputs in my MyLink now, even though nothing is connected. Is this normal?
I tried hooking up a generic USB hub with regular cables to split the signal from the HMI to both the front and inner USB hubs, but it didn't work.
So, it just seems that the front hub isn't working right to get the USB connection from the inner hub. The inner hub works fine with a direct connection to the HMI. Could this hub be defective? Is there anything else I could try to fix this?