I have quite a few recommendations for Multimedia authoring on Linux. Here's my top 5 for this category.

Audacity
An excellent audio editing application: simple interface for recording and editing sound clips. Also has some extra features like audio effects and tone generation. Can export to OGG natively and also to MP3 if LAME is installed.

Hydrogen
This is a very easy-to-use drum machine for Linux. It's actually useable without too much configuration and its interface is very, very intuitive. You can generate quite a few great rhythms with this one within minutes and also has a MIDI export option.

Blender
This is a very feature-rich 3d creation tool for Linux with the ability to create quite complex scenes and animation. The interface could be a little bewildering to a newbie, but it is quite a productive tool for 3d enthusiasts. Has a variety of export formats as well.

Cinelerra
This is a full-fledged video editing suite for Linux. I haven't actually used it, but it is quite popular in the Linux world among media enthusiasts. Deserves a place in the top 5 in this category.

ardour
A full fledged digital audio workstation for Linux. Again, like Cinelerra, I haven't used it, but it looks like an impressive piece of software and I guess it deserves a place here.

I'm sure there are a lot of very worthy contenders for the top 5 in this particular category, so feel free to debate this list and add your own suggestions as well.