Ubuntu Studio's selection of audio applications is impressive in both quantity and quality. There are at least three audio recording/editing applications: the solid and reliable Audacity; Time Machine, which has the unique capability of recording before you hit the record button (in case you make a really cool sound but are so involved that you forget to record what you're doing); and Ardour, which boasts features that rival those of commercial products.
MIDI processing and music-performance software includes the indispensable JACK system, a kind of Swiss Army Knife for routing audio and MIDI data. Software synthesizers include the Bristol analog synthesizer simulator and the multi-engine ZynAddSubFx. You'll also find several SoundFont-based systems, such as FluidSynth and Qsynth (the latter acts as a GUI front end to the former), as well as the GENPO (GENeral Purpose Organ) application. Ubuntu Studio also installs the robust Hydrogen drum machine, a percussion synthesizer and pattern-based sequencer.
Rounding out the musical performance software are BEAST (BEdevilled Audio SysTem) -- which is really a modular synthesizer engine and musical composition system in one package -- and the Pure Data (Pd) graphical programming environment, which can do everything from process MIDI and audio data to execute FM synthesis modules.
There's lots more, but available space cannot do justice to the full range of audio applications found in Ubuntu Studio. Even better, given that it is an Ubuntu distribution, you can use the Synaptic package manager to download all the standard Ubuntu applications you'll need when you're not using Ubuntu Studio to produce the next electro-trance hit.
Musix
Musix Linux is a labor of love by Argentinean musician Marcos Guglielmetti. Musix's heritage is a mixture of Knoppix, Kanotix, and Debian. The Synaptic package manager is installed, which opens Musix to the same wealth of downloadable applications available to any Debian system. Installation is tricky, however; I was unable to successfully install Musix on my test system, though I could run the OS in LiveCD form. Being a Knoppix-based system, however, it does have a boot option that will copy system files to a directory on the hard disk. This provides better performance than a LiveCD system, without requiring a full install that would otherwise erase what's already on the disk.
Though the name Musix implies an operating system bent solely on musical applications, Musix is pre-installed with tools, utilities, and packages that make it as usable as a standard desktop Linux. That is, you won't lack for a browser, a word processor, chat applications, graphics packages, and so on.