There are audio tools such as editors, mixers and synthesizers.

LMMS

A free alternative to popular (but commercial and closed- source) programs like FruityLoops, Cubase and Logic giving you the ability of producing music with your computer by creating/synthesizing sounds, arranging samples, playing live with keyboard and much more

With LMMS you can produce electronic music with your computer, so you don't have to buy expensive hardware for having great sounds and making cool music

Hydrogen

An advanced drum machine for GNU/Linux. It brings professional yet simple and intuitive pattern-based drum programming.

Sample-based stereo audio engine, with import of sound samples in .wav, .au and .aiff formats

RoseGarden

Is a professional audio and MIDI sequencer, score editor, and general-purpose music composition and editing environment

Beast

A powerful music composition and modular synthesis application.

It supports a wide range of standards in the field, such as MIDI, WAV/AIFF/MP3/OggVorbis/etc audio files and LADSPA modules

ReZound

A powerful graphic sound editor with mixer and effects.