Steps to reproduce:

1. Install Confluence in a machine that has no QuickTime installed
2. Attach sample_mpeg4.mp4 into Confluence
3. Embed it in Confluence page using Multimedia macro
4. Set 'Autoplay' On
5. Result: The mp4 file does not play

Looks like this is a problem with mp4 files in general, when QuickTime plugin is not installed in the system. This only happens when Confluence hosts the mp4 file. Saving the page to local hard drive (containing the multimedia macro) renders a proper behavior: The mp4 file is played despite not having QuickTime plugin installed.

Can't figure out what might be causing this as the source code is exactly the same, local vs Confluence, with the exception of where the mp4 file is being played from (in Confluence, it's played from the attachments link, while in the local hard drive, the mp4 file is loaded from the hard drive itself).