The best open source alternative to LMMS is Ardour, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to LMMS and 19 is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to LMMS are Rosegarden (Free, Open Source), Qtractor (Free, Open Source), Carla (Free, Open Source) and FamiStudio (Free, Open Source).
LNX_Studio is a Digital Audio Work Station created in the SuperCollider language. It has a powerful set of tools for creating music, all of which can be networked. Co-location gigs or real-time collaborations don't have to be in the same room.
"Sekaiju" is a free, open source MIDI sequencer software for creating and editing MIDI data. This software enables you to create music with your MIDI modules or synthesizers. Sekaiju also can be used as a powerful MIDI editor.