AV Linux is based on Debian Stable, with a low-latency kernel and lots of multimedia development tools. It seems to have a good reputation among experts in the field.
It comes on a 2.5GB DVD. There’s a pdf manual, with good instructions. It will run in 512MB, thanks to the use of the LXDE desktop, although more will be needed once you start video editing, for example.

Software is listed in the manual, available from http://www.bandshed.net/pdf/AV6Manual.pdf

There’s a lot of duplication (e.g. Cinelerra, Kdenlive, Lives, and Openshot), but no two users have the same needs. Codecs and Flash are installed. Those programs I ran from the CLI left no error messages and worked well. I didn’t actually install because it wanted to put Grub2 on my MBR, apparently with no option to load the resident distro, and I didn’t want the bother of restoring things.

I had only two reservations. Firstly, there’s no GUI tool to control multiple sound devices: this is typical of Debian, but inappropriate for a distro specialised for audio. Secondly, the release note quoted above describes this as a ‘final release’. The developer’s been feeling a bit overworked recently, but he seems to have cheered up a little, so this may not be the end. If the uncertainty troubles you, consider Ubuntu Studio.