How to fix off-sync sound and film when showing clips on an external monitor with M3 OSX Sequoia 15.4.1?

Thank you for the report. I will write some observations below, but first:

Have you tried playing back while in SAFE MODE? If it works correctly, it will be a strong indication that third party modifications are in play, and some of the things below are almost certainly involved.

---- Comments on the Etrecheck report ----

This machine has configuration profiles. These are sometimes installed in machines that are managed by a school or workplace, they do not normally exist in privately owned macs.

I don't know if these are playing any part in the issue at hand, but it's a possibility.

I am not familiar with "Snow Software" - it seems to be for managing machines for your organization.

You have apparently an antivirus from Panda, plus vpn stuff from nordvpn and fortinet.

All of this could be causing trouble. Antivirus are known to cause severe performance issues for no security benefit. VPN tools should only be used in a proper tunnel configuration to access a work or school network as instructed. Using a vpn does not offer the security that a lot of people think it does.

You also have Chrome and its background "keystone" agents - these run all the time and can cause trouble with the system videotoolbox. They are poison to video editing applications. I don't put it past them to cause trouble here.

Thank you for the additional information.

I use my mac with an external monitor at home and with a projector when teaching, both connected via HDMI. I do not see this kind of delay. I only play videos on the projector very rarely, but play them on the external display in my home office almost every day. I have not experienced this sort of problem.

Just now, I made a test Keynote presentation, added a short video to a slide, and played it back from the presentation. Again, no hiccups or delays.

I wonder if there is something with the cables, displays or - perhaps more likely - some software conflict. Since you are experiencing the problem after upgrading the OS, it is quite likely that some system extension or daemon from before is not working correctly in Sequoia.

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

Do the films play correctly:

• when using the internal display only?
• when the external or projector is connected but film is played in the internal display?

What films are these - which codec, and frame size?

What program are you using for playback - Quicktime Player, VLC, something else - and does the program make a difference to the end result?

I can surely understand how this is a vexing issue, and I hope we can work towards a solution. I have not experienced this issue myself.