You can now test MP3 and MP4 files with Design surveys - UsabilityHub updates
Sample MP4 video files
How to Repair Corrupt Videos - MiniTool Video Repair Manual
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Need Help: Unable to upload MP4 videos on web or app, got error message: “Your video file is not compatible. Please see tips for uploading videos.” - Media APIs - Twitter Developers
Audio/video files not being reproduced (mp3/mp4) - Fault reporting - The meet.coop Forum