I have a site with m4v files that played before the upgrade to 10.6.7, now just sound

http://www.myfarrah.com/ACT7.php works here. Launch Disk Utility, repair permissions, restart, and try again.

Are you using Safari or Firefox? It runs as expected in Firefox, but in Safari I too get just the sound, and going to the m4v link causes the movie to download rather than play (it downloads fine and plays in Quicktime once downloaded). I checked some of my own m4v movies, and they were playing fine in Safari. For one embedded in a complex web page, see here:

http://www.pinkmutant.com/articles/snowleo.html

The next m4v has its very own very simple page, with the movie called using an embed tag with various options set (you can see the details by going to View->View Source) and also plays:

http://homepage.mac.com/francines/Movies/duomo.html

There was a problem with a Safari plug-in misbehaving very early in Snow Leopard, but I no longer remember the details. But see this archived discussion:

https://discussions.apple.com/thread/2158092?start=0&tstart=0

It was a rather complex problem, but I had thought one of the updates fixed it.

Francine

Your html code is woefully out of date and will not work with Internet Explorer.

http://developer.apple.com/internet/ieembedprep.html

Sorry, was away for awhile, but it works w/Safari 5.0.5. For the OP, glad you sorted things out.