Stopping ffmpeg being downgraded
 

I have installed ffmpeg from jessie-backports
"sudo apt-get -t jessie-backports install ffmpeg"

but it would always get downgraded when running "sudo apt-get update; sudo apt-get upgrade"
to version in deb-multimedia.org repo.

$ apt-cache policy ffmpeg
ffmpeg:
Installed: 7:3.2.4-1~bpo8+1
Candidate: 10:2.6.9-dmo1
Version table:
10:2.6.9-dmo1 0
500 http://www.deb-multimedia.org/ jessie/main amd64 Packages
*** 7:3.2.4-1~bpo8+1 0
100 http://ftp.uk.debian.org/debian/ jessie-backports/main amd64 Packages
100 /var/lib/dpkg/status

I have tried pinning before, but could not get it to work ..
What would be the "Debian" way to stop this, and keep the version from jessie-backports?

Comment out the http://www.deb-multimedia.org/ in your sources.list and only enable it when needing to install a package from it. Otherwise try the pinning again and see if you have better success.

The problem here is that deb-multimedia appears to have changed the epoch of ffmpeg and many of its dependencies from the normal (current Debian epoch for these packages) 7: to 10:, so they will always be considered as the candidate version.

Perhaps you might want to contact deb-multimedia and ask them why they've done that.

Edit: see "Versioning" here - https://wiki.debian.org/DebianMultimedia/FAQ

It appears that deb-multimedia do this intentionally in order that their packages take priority, even when the actual underlying version number is less than the current Debian-packaged version.

Try
after you have The Version You Like installed.

mark
update
check.

No editing. :)