Hello @Matthew_Ganes, I have investigated your presentation using a Mac and I was able to reproduce the videos without any issues both through the application and also in the downloaded presentation.
When using the desktop app, I would advise you not to have any other programs/applications open meanwhile, as this will improve the performance.
You can also try to delete the following destination folder without losing any of your content (please, assure that all the presentations are synced online):
MAC: /Users/USERNAME/Library/Application Support/com.prezi.pitch.desktop/UID/content
(please, note that every time you use the desktop application the files will be downloaded once again.)
Let us know if this could help!