Audacity for Linux is a good free sample editor.
Skale also has a linux version, pretty much on par with it's Windows Counterpart. Unfortunately, it's a little dead over at their website.... They do promise a new version "When it's Ready", but they say that about Duke Nukem Forever as well...
Some people like Ardour, for loop/sample-based music. Rosegarden is similar in purpose.
I'd check out
http://linux-sound.org/ to see what's out there, even if the page is a little dated at times....