Here is the debmirror command line for this repository :

debmirror <mirror-directory> --host=www.deb-multimedia.org --ignore-small-errors --diff=none --dist=stable,testing,unstable,experimental --arch=amd64,arm64,armel,armhf,i386,kfreebsd-amd64,kfreebsd-i386,mips,mipsel,powerpc --root=/ --method=http --section=main,non-free

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Of course you need to change <mirror-directory> to your local directory. You can also changes the --arch and/or --dist options if you don't want to mirror all the repository.
The --ignore-small-errors option is here because oldstable doesn't have armel and mipsel architectures. If you don't want oldstable you can remove this option.
Beware the repository size is 15 GB (11/04/2020).

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Also a change need to be done for the gpg key.

From /usr/share/doc/debmirror/NEWS.Debian :

The Release.gpg check has been rewritten to use gpgv, the same
method as used by apt and debian-installer. As a side effect the
keyring file has been changed to ~/.gnupg/trustedkeys.gpg.
Existing setups must copy their pubring.gpg to trustedkeys.gpg or
import the relevant keys there. This allows to only have the
Archive keys in trustedkeys.gpg while still having a pubring.gpg
for other use.

You can also use rsync :
rsync://www.deb-multimedia.org/deb