there are a few choices ardour is pretty good with a qbase interface also gnusound is similar in functionality but they are both realy writen for gnome, jokosher is a good kde composer alowing you to both record and compose using samples the interface however is a little simpler and is more for the garage band enthusiast.
if not check out ubuntu studio instal it along side your kde and select it at login
heres a few to look up: Rosegarden lets you fiddle with score so you might like this one. Then theres qtractor and muse. For audio try ardour, LMMS for a fruity loops vibe..hydrogen for drums