In our experience so far, this type of behavior almost always means one of two things:
- The customer has some sort of external speaker (Bluetooth, etc.), and the sound is delayed due to it being transmitted to a speaker while the video plays immediately. Hooking the external speaker up via an audio jack versus over Bluetooth can sometimes help with this. For troubleshooting, if you are using external speakers, I would see if it plays in sync using the computer speakers versus external if possible.
- Our customer has an external program like anti-virus or a browser plugin or sound enhancement software that is interfering with the sound being played immediately. Using a "Private Window" in Safari or Chrome's "Incognito" may help with this, as most plugins are disabled in private browsing.
Trying a different Web browser may help as well, not necessarily because of the Web browser itself, but because something set up in or added onto your current browser may be causing an issue that won't be found in a clean browser install.