MLED, aka the Microlinux Enterprise Desktop, is Frenchman Nicolas Kovacs' attempt to bring together various bits and pieces of the Slackware community, including Slackbuilds, slackpkg+ (which I confess I've never heard of until now) and more to create what he calls a "full-blown production desktop."
Yes, that includes multimedia codecs.
You get MLED by installing Slackware, then importing "tagfiles" (first time I've heard of this concept) and doing more than a bit of configuration, choosing KDE, Xfce or MATE along the way.
It's not as easy as a full-blown ISO but not as hard as finding all the bits on your own.
Kovacs talks about why MLED is based on Slackware here, and I agree with pretty much everything he says.
If you're looking for a long-term-support distribution with extremely conservative underpinnings, Slackware is a compelling choice, and it looks like MLED will get you from zero to desktop that much more quickly than assembling the bits on your own. I'd prefer this to be more automatic, but those are the Slackware-fueled breaks, I guess.