Audacity is a cross-platform multitrack audio editor. It allows you to record sounds directly or to import Ogg, WAV, AIFF, AU, IRCAM, or MP3 files. It features a few simple effects, all of the editing features you should need, and unlimited undo. The audio I/O uses PortAudio, which fully supports OSS, Mac OS X CoreAudio, and Windows WMME, and can be compiled with support for ALSA and Jack.
Ecasound is a software package designed for multitrack audio processing. It can be used for simple tasks like audio playback, recording, and format conversions, as well as for multitrack effect processing, mixing, recording and signal recycling. It supports a wide range of audio inputs, outputs, and effect algorithms. Effects and audio objects can be combined in various ways, and their parameters can be controlled by operator objects like oscillators and MIDI-CCs. A versatile console-mode user interface is included in the package.
MuseScore lets you notate and compose music easily with your keyboard, your mouse, or a MIDI keyboard. You can print beautifully engraved sheet music or export it as PDF or PNG. You can play your score, transpose it, and save it as audio or MIDI file. MuseScore also supports MusicXML to let you share your composition with other scorewriters. MuseScore is often advertised as a cost effective alternative for Sibelius and Finale.
GLAME (GNU/Linux Audio Mechanics) is meant to be the GIMP of audio processing. It is designed to be a powerful, fast, stable, and easily extensible sound editor for Linux and compatible systems. It has full support for non-destructive editing including undo/redo and applying LADSPA effects. Its supported platforms are Linux, BSD, IRIX, and OS X. It uses guile and libxml, and the GNOME libs available is highly recommended. MP3 and Ogg files can be processed if libmad and libvorbisfile are installed.
mpgedit is an MPEG 1 layer 1/2/3 (MP3), MPEG 2, and MPEG 2.5 audio file editor that is capable of processing both Constant Bit Rate (CBR) and Variable Bit Rate (VBR) encoded files. mpgedit can cut an input MPEG file into one or more output files, as well as join one or more input MPEG files into a single output file. Since no decoding or encoding occurs during editing, there is no audio quality loss when editing with mpgedit. mpgedit features command line, curses-based, and graphical interfaces. The curses and graphical versions feature an interactive playback mode for fast selection and verification of edit begin and end times.
GNUsound is a sound editor for GNU/Linux. It supports multiple tracks, multiple outputs, and 8, 16, or 24/32-bit samples. It can read and write many audio formats through a number of supported file format libraries (such as AudioFile, libsndfile, and libmp3lame), and it can extract audio from a variety of video files through FFmpeg. It supports OSS, ALSA, or JACK for playback/capture, as well as a large number of high-quality audio effects through the LADSPA plugin architecture.