Embedded QuickTime files not playing sound

The QuickTime browser plug-in has ended support for dozens of legacy codecs in 10.9.5, and higher.

When you downloaded the file it was "converted" by QuickTime 10.4 so that's why you heard the audio track.

I am having this same issue. I am a substitute teacher and sometimes teachers lave audio instructions on the management system we use, via web access. These audio links used to play using Quicktime, in my browser. Now, they simply appear to play, but no sound is audible. I am on windows and have tried it on Firefox, Chrome, and IE. If I roll-back to a previous Quicktime version, would this help? Currently using version 7.7.6

Is this a Yosemite issue? Not being able to play older QuickTime 7 files properly? I'm not sure what else the issue could be since everything seems to work fine in 10.7, 10.8, and 10.9 (unfortunately, I have been unable to test this in 10.10 or 10.10.1). Maybe I'm overcomplicating a simple issue? Any help would be greatly appreciated.

Some very interesting observations which peaked my own curiosity due to on-again, off-again nature of the playback. I suspect this is either a "bug" or represents content handling changes that are currently in the process of "evolving." Not a web site programmer so I used iWeb to run some contrast-comparison tests on how audio was handled by the plug-in software when directly accessing the content by its URL and via the page embed strategies. My results mirrored yours in that:

1) All legacy codecs tested (A-Law, IMA, QDM2, QPV, and u-Law) in an MOV file container played correctly in the QT 7 player.

2) The same files all played correctly when content was accessed by the plug-in software directly using the URL reference.

3) None of the files containing legacy audio compression formats would play audio using the plug-in software using a web page embedding strategy.

4) An MP4 (H.264/AAC) version of you original clip played correctly in all instances.

5) Replacing your QDM2 audio track with an AAC audio track in the original MOV file allowed the file to play correctly in all instances.

Based on these observations, I suspect the problem is in the way the software is currently programmed to handle (or rather ignore) the embedded strategy content. However, as you indicated, it is probably time to update your files. Not only did the H.264/AAC version of your original file play in all instances, but it used less than 45% of the file space/bandwidth for the settings I employed as compared to your original "0706_split_brain_red.mov" original file.

If I roll-back to a previous Quicktime version, would this help? Currently using version 7.7.6

Unknown. Would likely depend on how the changes were accomplished—within the QT structure or the plug-in software. A better approach would be use a more modern audio compression format if possible. I.e., it is unclear if the software you referenced records the messages using a "legacy" audio format or if it manages audio messages recorded using other software. In either case, using a more compression format like AAC (if available) would appear to solve this problem.

So, I uninstalled 7.7.6 and rolled back to 7.7.5

Fixed the problem. Audio working again.

So, I uninstalled 7.7.6 and rolled back to 7.7.5

Fixed the problem. Audio working again.

That's good to know for the Windows side. Unfortunately, the Mac side may require rolling back the OS also. Also went back and re-ran all of my earlier tests. Combining legacy video with modern audio (not your problem but original poster's) now resulting in inconsistent results. Sorensen 3 Video with AAC in MOV sometimes plays using direct URL access and sometimes doesn't. Luckily, the file always seems to work with embedded pages. "Curiouser and curiouser."