Music composition and audio editing software can be quite expensive. There are professional open source alternatives to expensive commercial applications. Open source applications do not get the exposure as commercial applications and may not have the same features or operate in the same way. Deciding if open source software is appropriate is the same as deciding what commercial software to buy, the features of the software need to meet the needs of the user.
I enjoy writing music in my spare time and find myself editing audio files on occasion as well. I have installed and use two music composition applications and two audio editing applications.
Rosegarden is an application that can be used to compose, edit, remix, etc. music in an environment that feels similar to commercial applications such as Cakewalk and Cubase.
Rosegarden can be used to record and play audio, MIDI and soft synths. It has editors that someone would expect in a music composition application, such as the notation editor, matrix editor and event editor. The notation editor is for editing notes on a staff. The matrix editor is mainly used for percussive instruments and the event editor can be used to fine tune midi controller events.
There are many plugins for Rosegarden and plugins for other applications can be used such as VST plugins.
I have never been a fan of music composition applications that use a piano roll editor as the main editor, I have always liked editing music using a notation editor. The sound that LMMS is capable of let me look past the editor and really enjoy it. Linux MultiMedia Studio is packed full of software synths, can trigger sound modules with MIDI and can import different plugin types. There are plenty of effects with real time recordable controls. LMMS is the all in one studio application that does not have a steep learning curve and has impressive results.
[audio:http://blog.mpshouse.com/wp-content/uploads/2011/08/The-Search.mp3|titles=The Search]This is something I put together in LMMS (Click Play)
This is the audio editor I use for editing audio files. If anyone remembers an audio editor called CoolEdit, the user interface in Audacity is quite similar. Audacity has many filters and plugins and will use existing audio plugins on your computer. On my computer Audacity has found and is using 58 audio plugins.
Ardour is a professional hard disk recording application that can be controlled with a MIDI interface. Ardour has been embedded in professional hardware for musicians and recording engineers. Audio can be recorded on separate tracks and played together, this is great for recording and editing musicians tracks separately. Ardour has an impressive feature list such as unlimited audio tracks and buses, unlimited pre- and post-fader plugins and automatic track delay compensation.