Q: QuickTime no longer plays certain MP4 and MOV files

I too can no longer play mp4 video files - mine were generated with compressor wheras ProRes422 was the source.  QuickView shows a zero k file, get info shows the proper size, QuickTime X wants to convert it to mov, QuickTime 7 Pro plays fine, the same with VLC.  They were are all good prior to Mavericks.

MacBook Pro Retina Display.

Me too.  iTunes doesn't play my older mov H264 files and QT wants to 'convert' them, which is a pain.  QT also wants to 'convert' my .wmv files.  That process takes quite a long time and is frustrating when you're showing a movie clip to a business audience.

I've had to use VLC for almost ANY video file I have...thank god someone has their heads in the game and their ears open.

I too find that my older Quicktime 7 Pro will still play the files without issue...no conversion at all.

Odd that this is 'progress.'  I can understand the need to advance the software compression with time but to remove prior functionality from a native video player that is affecting iTunes which is a big money maker for Apple confuses me.

I have found that if I uncheck 'Streaming hints enabled' in Compressor my (newly created) .mp4 files will open in both QuickLook and QuickTime X.  This does nothing for existing files, I know — but maybe a clue to the problem and hopeful solution.

Where is the Streaming hints enabled checkbox?  And is compressor a seperate ap or just part of QTX?

Sorry James, I may have added confusion.  Compressor is a separate (Apple) application used to compress video into a variety of formats, sizes, bit rates, etc.  In my case, I am creating and editing videos in Final Cut Pro and sending them to Compressor for output.  Compressor gives more options over the standard export options in Final Cut Pro - and it works in the background.

If I use the settings I always have, QuickView no longer works with those files, nor will QuickTime open them without converting to .mov.  Now, if I disable 'streaming hints' in Compressor - using my otherwise same settings -the files work with both QuickView and QuickTime X, as they did pre-Mavericks.

Aha!  Thanks for the clarification.  I don't use Final Cut Pro but if I ever do...LOL. 

I belive Quicktime has removed the ability to use third party codecs as mentiond in other threads.

https://discussions.apple.com/thread/5515994?tstart=60

It is why Quicklook or Finder doesn't generate previews for wmv's etc. It will only work with a specific set of codecs. in the past you could install Perian, or Flip4Mac to add extra support to the codecs across the system.

VLC is the easiest way to play most files, try that.

https://videolan.org/

As ever, if it bothers you leave feedback

http://www.apple.com/feedback/

You may have better luck in the Quicktime threads.

https://discussions.apple.com/community/mac_os/quicktime

This post by martbr explains the situation well…

https://discussions.apple.com/message/23530041#23530041

Try Quicktime player 7 (it's better if you have a QTPro 7 code)

http://support.apple.com/kb/DL923

It seems to support everything that it did before Mavericks (including Flip4Mac & Perian codecs), what is different is that the OS uses AVFoundation not Quicktime, the former doesn't support additional codecs therefore there is little hope of getting Finder previews for files with unsupported codecs.

Hint tracks were the culprit for me, too.

To remove hint tracks from existing files you can use the excellent "Subler" MP4 muxer/editor (OS/free).

Just delete the hint track(s) in the list, save, voila.

https://code.google.com/p/subler/

Thanks for the links, Drew, there's a lot of good discussion there.  I can understand why some video files are having trouble (wmv, divx) - but my files are using the H264 codec, very much an Apple standard.

Subler did it!  Subler fixes my existing videos.  Thank you, Block Rocker!

H264 isn't the only factor.

There are several H264 'profiles' and other minor variations that means files won't play when you think they should. Throw in audio codecs & you have a mine field for compatibility.