The Dropbox mobile player doesn't support Opus format yet. This is a pain, because many of the tutorials and demos we've collated in these here folders have been extracted from WhatsApp, which uses Opus as its format for voice messages. We've experimented with converting the files to other formats, but haven't been able to find anything that sounds as good as Opus without significantly increasing the file size of each tutorial. That's a no-go, because many subscribers are using Dropbox's free plan which has a strict size limit.

Luckily, there is a solution.
Grab VLC from whichever flavour of app store your device uses. It's free, it's accessible, and it supports Opus format.

Here are a few hints to help get you started with iOS and Android.

If you're going to use these files often, then probably the best approach is to open VLC, double-tap the "Network" tab which you'll find at the bottom of your screen. Double-tap "Cloud Services" and sign into your Dropbox. Once done, you'll be able to browse and play all of the content herein without needing to faff about exporting one file at a time.

If you'd prefer to manually export individual files for some reason, here's how it's done.

Using iOS:
Swipe down on a file you'd like to play until you hear VoiceOver say "Export", then double-tap.
The iOS share sheet will open, and at the bottom of your screen, you'll find a button labeled "Open In..." which you'll also need to double-tap.
Dropbox will export the file, and you'll be presented with a list of apps that you can send the file to. The option you're looking for is "Copy to VLC". Note: you might need to double-tap "More" to reveal that option if you have a lot of apps installed.

Using Android:
Swipe right on a file you'd like to play. When you hear your screen reader say "More options", double-tap.
Double-tap the "Export" option which you'll find at the bottom of the resultant menu.
The Android share sheet will open, and in there, you'll find an option to open in VLC.

Shout out to Kyle for suggesting VLC, and Dennis for the Android instructions.

Enjoy!