All,
While onsite to support a client with a Vaddio AV Bridge MatrixMix that wasn't passing audio from their iPad or MacBook I found that using the client's new Apple dongle model # A2119 (EMC3279) would pass HDMI video, but would not allow audio to pass from my laptop using a Mini-DisplayPort/HDMI adapter. I know the thread is about the Apple dongle, but bear with me.
After confirming the configuration of the device I chose to test with my older Apple dongle model #A1621 (EMC2877) and audio passed correctly.
It appears the client's new Apple dongle may have an issue in the updated iOS that locks up the digital audio and the only way to get audio to pass was to reboot the MatrixMix.
Once I locked it up again using the client's Apple dongle I confirmed that no audio would pass from any type of adapter and then cleared the issue using the A1621.
I even used the client's 3rd part USB-C adapter which initially worked correctly, but once the A2119 was used it would prevent audio from passing and wouldn't work until I used the A1621 dongle or rebooted the MatrixMix.
Once the older A1621 dongle was used and audio passed correctly I could revert back to the new A2119 dongle or other dongle and everything worked as it should.
The Apple store doesn't list the model number of the current USB-C Digital AV Multiport Adapter, so I wouldn't count on purchasing it from them unless they can confirm availability to the A1621 dongle.
I hope this helps, but if not, my issue may have been specific to the Vaddio AV Bridge MatrixMix.
It may also be relevant to reboot the TV/Display/Video component after you have the Apple device connected and powered on, but I didn't attempt that while onsite.
Good luck!