Unfortunately the DJI Goggles do not provide a media player facility - which is too bad since they'd be a very good viewing device. You can insert an SD card but not make much use of it besides storing screenshots and downloading videos from a Mavic wirelessly (and very slowly) for later playback. It has been found that you can place custom videos at the same place and with the same naming convention as those downloaded files and they will play, but without sound (since DJI aircraft videos do not have a sound track DJI probably haven't bothered with sound at all).
The goggles however come with 4 preloaded demo videos that play with sound, so we're going to exploit that feature and inject our own “demo videos” in the same location and format.
The caveat is that there is no way to scroll the demo videos, only play/pause.
You need to create:
All 3 files need to share the same name, e.g. MyVideo.MP4, MyVideo.wav, MyVideo.jpg.
The demo videos are stored in a 2GB partition in the Goggles' internal storage. The 4 original demo videos take about 730MB, so if you leave the original videos there you have about 1.2GB to play with.
The folder happens to be accessible through the built-in FTP server, in blackbox/demo_videos. To reach it power your goggles up, connect the USB cable to your computer, and using an FTP client connect to 192.168.42.2 (no user/password required, any will do if your client wants one). Note that you need a working install of DJI Assistant 2 on your computer for the proper RNDIS driver to be installed, if Assistant can see your goggles this will work. Browse to the folder, you can easily add and delete files there, but currently it isn't possible to read any out without root so it isn't possible to backup the originals, if you delete them to make more space and later want to restore them you will need to obtain them from someone else who made a backup using root. All you need is put your triplets of files in this folder through FTP, then disconnect and reboot the Goggles, the videos will show in the Demo Videos pane. You can delete from FTP when done and put others on.
Unlimited as in whatever the size of your SD card is of course.
cp -r /blackbox/demo_videos /sdcard/
mv /blackbox/demo_videos /blackbox/demo_videos_orig
ln -s /sdcard/demo_videos/ /blackbox/demo_videos
After the reboot you should see the 4 original demo videos as usual, but the big difference is that they are now taken from the SD card. You can take the card out and put it in a reader (or access it through the goggles' USB port), dump all your videos on there and they will appear next to the originals… which you can delete or move away from the card if desired, you still have them on the goggles' internal storage anyway. The list will scroll if you have more than 9 videos.