Consider 64 Studio, which packs a whole slew of free and open source multimedia (music and video) content-creation applications such as Ardour, Hydrogen, and JAMin into a 64-bit Debian-derived Linux distribution. The 64-bitness is pretty deliberate: With applications like audio and video creation, having unimpeded access to as much memory as you can is always a good idea. Upgrades are provided through the apt-get package system, so you can keep the whole system current with only a couple of mouse clicks.
The distribution itself is free, and the creators are preparing a technical support purchase option along the lines of what Canonical charges for Ubuntu (which is about $250 a year per user). One major missing element: Because of patent issues, no support for MP3 is shipped with the distribution, but there are workarounds suggested for this in the documentation. There's no live CD option, either, so you do need to commit a partition to trying it out.
But if you're curious about building a Linux workstation specifically for multimedia creation and want a distribution that has most of the important stuff already there, give this one a whirl.