> in /etc/apt/sources.list
> is by default already a deb-multimedia repo inserted? If yes, why
> the old squeeze?
As far as I know, we do not add such repository by default. If we do,
it must be some old code that should be updated to use wheezy.
> (I just do not understand why I have a deb-multimedia squeeze line
> around, maybe I've messed the file)
Check the history using
cd /etc/apt; git log -p sources.lost