In real world, there are only a small portion of operating system who can handle ALL multimedia formats. It's not impossible to do that, but they will end up with legal problem, which can bring them problem. So most maintainer doesn't include packages that would bring them problem.
Slackware is no exception. Full installation of Slackware will not be able to play *ALL* multimedia formats, but with some configuration and installing new packages, you will (hopely) have a full-featured multimedia Slackware which is ready for personal use (not a professional, but it can be configured to be like a professional if you like).
Darrell Anderson has wrote an interesting article called A Full-Featured Multimedia Slackware. In that article, he wrote his experience of turning Slackware into multimedia-ready . For you guys who wanted to have better multimedia support on your Slackware, check the article. Also give your feedback if you have better reason to include another application which can enrich the article. You can do so at LQ Forum.