LMMS is a free cross-platform alternative to commercial programs like FL
Studio, which allow you to produce music with your computer. This includes
the creation of melodies and beats, the synthesis and mixing of sounds, and
arranging of samples. You can have fun with your MIDI-keyboard and much
more; all in a user-friendly and modern interface.

jack-audio-connection-kit, fluidsynth + fluid-soundfont are optional (but
are highly recommended) - stk, portaudio, pulseaudio also are optional.
