LMMS
LMMS focuses on making music with beat and instrument tracks.
What it does
LMMS builds songs with beat patterns and instrument tracks.
When to use it
For a first soundtrack sketch, repeat a beat pattern and add chords.
Visit LMMS official site
Amped Studio
Amped Studio focuses on browser-based recording and multitrack production.
Ardour
Ardour focuses on recording, composition, and mixing workflows.
Audacity
Audacity focuses on waveform editing, recording, and audio effects.