William Morder via tde-users wrote:

> Like yourself, I imagine, I hate change, hate the need to rebuild my
> system that was working just fine. But once that release is no longer
> supported, it doesn't make much sense to keep using it for years beyond
> its security updates; whereas Buster/Beowulf LTS will be supported for --
> how long is it? -- 5-7 more years.

Yes indeed - I can use a VM as a guinea pig, but not my own PC, so usually I
would wait a bit after a release updating my production system(s).
The good thing is that many people use testing and report back issues, so
that these issues get solved.
I do not see a reason not to upgrade to stable couple of months before it
becomes old stable.

Just install and try mplayer from the command line - I am sure it was
working fine with mp4 files under stretch.

read this: https://wiki.debian.org/MultimediaCodecs