No surround sound when using USB-C multiport A/V adapter

I have number of .ts files (mostly TV episodes) that I play over HDMI to a connected receiver or projector. I typically use VLC or Quicktime to play the files.

On my previous generation MacBook Pro, I just plugged an HDMI cable directly into the HDMI port on my computer and it worked fine: full resolution video and surround sound through my A/V reciever.

On my new touchbar MacBook Pro, I am using the USB-C Digital Multiport adapter (since there are no HDMI ports on the computer anymore). Video outputs fine, but the audio is messed up. None of the vocals/dialogue can be heard. It seems that the center channel information is not coming through. I tried to play the same files through an Extron scaler (part of the projector system at my school). The audio in that room is stereo and the center channel audio still cuts out. All I can hear is the background audio (crowd noise, wind, etc.).

I looked at the audio tracks of these files more closely and they are all AC3 (5.1 channel).

When I look at the audio output choices on my computer with the adapter plugged in, it reports as Display Port, not HDMI. I'm guessing that the native output from the USB-C port is therefore DisplayPort and the adapter is converting it to HDMI. Perhaps this is what is mucking with the audio?

I'm waiting for direct USB-C to HDMI cables to become available so I can use HDMI alt-mode and don't have to use an adapter. Until then, is there anything I can do short of converting the file to down mix to stereo? Is there some sort of setting I can choose in VLC or Quicktime that will at least play the file in stereo when I output it to the projection system in class so I don't have to worry about losing audio during a presentation? Unfortunately, when I preview the content at home, the audio works fine--but that's because I have the computer plugged into a Thunderbolt 2 display with audio. I hate not knowing if it's going to work when I get to the classroom and plug in via HDMI.

Thanks.