Audacity is totally free and is a really easy to use sound recording and editing software. Before I got into Ubuntu five years ago, I was in the music biz. I did (and still do) a lot of audio work and really wished I had a free audio program, heck a free pro tool like Audacity, which can do a ton of great audio editing and effects.
Here are some great Audacity links to get yourself acquainted...
Audacity Quick Guide
One thing great about Audacity is the sheer number of plugins available. For us Ubuntu users, you should be able to open the Synaptic Package Manager (System > Administration) and do a search on "swh-plugins" to install Steve Harris's library of LADSPA plugins for Audacity. I count over 150 plugins such as amp, overdrive, comb filtering, eq, flanger, chorus and lots more. If you hunt around the net, you might even come across some LADSPA plugins not in Steves library. Here are a few I've found that I like...
- compress.ny (from here http://pdf23ds.net/software/dynamic-compressor/ )
To install those, right click and save them. Then, go to your terminal and type in "sudo nautilus". It will bring up a super user file directory and you will want to place those "ny" extension files over in the folder "user/share/audacity/plug-ins/". Be very careful not to delete anything from your folders while in super user mode! Bad things can happen :) After placing those plugins in the plugin folder, exit out of the folder and out of the terminal. Open up Audacity and you should have those new plugins added to your plugin list.
Lastly, there are a TON of video tutorials on Youtube walking you through different how to's in Audacity. From simple audio layering, making a podcast, to easily removing vocals from an music track... it can be done and easy with Audacity.