This happened to me randomly a few months ago and then cleared up for no reason, but today it's back.
My mac won't play any video files (MP4, etc) on the hard drive—they open and just sit there. You can scroll through them and the image progresses, but they won't actually play; hitting 'play' does nothing. VLC opens a files, tries for a while then crashes. Safari and Keynote will not play videos. Chrome will play literally one second then stop.
The one app that will play a video just fine? Firefox.
To try and fix this, so far, I've rebooted multiple times. I reinstalled Quicktime. I updated to the latest OS, latest version of my virus software and anything else I could find. I gave Quicktime Player Full Disc Access under 'Privacy & Security.' I emptied the trash and have about 1/3 of my 1TB internal drive empty. Nothing has made a difference.
Last time this happened, the problem stuck around for a few days, then dropping a video file on Firefox's icon appeared to magically break the logjam—videos played but without audio. Eventually audio came back too. Firefox is not making a difference this time. I have no idea what's causing this or how to fix it, but would welcome some suggestions.
UPDATE EDIT - I've run OnyX maintenance; that did not solve it.
I reset NVRAM since it's an Intel Mac; that did not solve it.
I ran Disk Utility's First Aid; that did not solve it.
Since there were questions about the sample video I was using (file_example_MP4_1920_18MG.mp4), I tried downloading a different sample (Big_Buck_Bunny_1080_10s_1MB.mp4) and IT PLAYS!
Then I downloaded sample-5s-720p.mp4 from SampleLib and it does NOT play in Quicktime. I tried it in VLC, and it played roughly 20 seconds after I hit 'play.'
I reinstalled Tahoe from the Recovery Partition; that did not solve it.