MuseScore is a free and open source cross-platform WYSIWYG music
notation program. Its features include:

* WYSIWYG, notes are entered on a "virtual note sheet"
* Unlimited number of staves
* Up to four voices/staff
* Easy and fast note entry with mouse, keyboard or midi
* Integrated sequencer and FluidSynth software synthesizer
* Import and export of MusicXml and standard midifiles (SMF)
* Platform independent code for Linux, Windows and Mac

NOTE: You can replace jack-audio-connection-kit with jack2