Note, starting any of while doing anything else with audio is probably a poor choice. At least in Linux. Because Linux audio is still slightly Lovecraftian.
Tools Thaj suggested:
LMMS,
Ardour,
Qtractor,
Fluidsynth,
Hydrogen,
Luppp,
Guitarix,
Rackarack
https://www.youtube.com/watch?v=MPmkd0fgiLU