Why can't I play MPEG-4 files ?

these are old files from a DVD. source is unknown. I have 7 or 8 (each up to 255 MB) of them so I need to 'transcode' them I suspect. Where is this procedure described ?

Sorry! Went to bed early last night. Didn't see your response until this morning.

As to the source, the files appear to have been encoded by non-Apple software/devices. Unless you can get your "source" to re-encode the files, then you only have two options...

1. Play them in a legacy compatible Apple or third-party media player (e.g., QT 7, VLC, MPlayer, etc,) or
2. Transcode the data so as to be compatible with the more modern QT X media player.

As previously indicated, Mac OS X versions Mavericks through Sierra can access to the "Convert" structure that is built directly into the operating system via the Finder. Simply select one or more files you wish to convert in a Finder window, "Control-click" one of the "selected" files to open the Finder "Context" Menu, select the Services "Encode Selected Video Files" option, and select the target settings for transcoding all of your "selected" files. In my case, I used the "720p Higher Quality" built-in system structure setting to clip/limit the maximum frame rate used in your source videos to a 30 fps output which makes them compatible with the QT X media player. This option also switched the encoding Profile from the older, higher data rate "Baseline" setting to the more modern "High" setting which offers more built-in context adaptive features that retain the "visual" quality of your source file at a lower video data rate. (I.e., your new file is about 40% smaller than your source file but retains its original "visual" quality.)

NOTE: HandBrake can be used as an alternative to transcode such problematic files on any QT X based Mac OS X system since it offers a "Peak Framerate (VFR)" mode that clips/limits encoding to a user selected maximum frame rate.

If you have a problem understanding either method to fix your files, I can probably create a "quickie" screen capture tutorial (when not busy encoding something else) that demonstrates how to use either workflow.

It isn't obvious to me how to attach the .mp4 file to this reply... drag and drop didn't work and I don't see any 'add attachment' tab. A little help please...

Small files can be e-mailed to individuals. However, most users simply post the files to a server or online service like Dropbox and then post the URL for the file here in the forum.

Why can't I play MPEG-4 files on my iMac running Sierra ?

Based on the information given, either something is wrong with the file or something is wrong with your system and/or player. Provide more information or a copy of the file for examination.

Why can't I play MPEG-4 files ?

File audio bit rate/sample rate combination is non-standard if it was created by an Apple device. However, this does not appear to be the main problem since it played fine in QT 7. (I.e., since it would not open in the QT X player, I am inclined to suspect the file is possibly being error trapped as a security issue.) In an effort to get around the problem, I accessed the "Convert" structure embedded in the Mac OS via the Finder and converted the file directly to a 720p M4V file without having to open the QT X player. This new file seems to open in the QT X player without any issues. Here is a link to the file: http://downloads.walker4.me/Downloads_files/AND%20THE%20HOME%20OF%20THE%20...%20 BRAAAVE.m4v

ADDED:

Think I may have found the main problem. Your MP4 was encoded as a 720p VFR Baseline @ 3.1 file. This means your max frame rate is limited to 30 fps. However, while your VFR average frame rate is only 29.806 fps (which is acceptable), it varies between a minimum of 8.197 fps to a maximum of 33.333 fps (which is not acceptable). As a result, the QT X player refuses to open, load, and play the file.

Basically, unlike many third-party media players, QT X will not play files that are not "standards" compliant. If you both want to use your current encode settings and want the files to play in the QT X player, then either encode in a constant frame rate mode (if available) or encode using a lower/limited frame rate (if available). Otherwise you will need to transcode the files post recording to make them QT X compatible.

Several converted and opened but a couple didn't. Many thanks for your extended help.

Files produced by third-party apps can have "standards" compliancy issues other than the ones already mentioned. If you have HandBrake already installed on your system, I would suggest your try it using one of the "General" or "Device" targeted presets to see if that "fixes" the still non-playing files.