Some links to open-source audio software projects that Scot Gresham-Lancaster sent me a few months ago:
Digital Audio Workstation software.
a low-latency audio server. Allows multiple applications to share audio devices and share audio amongst themselves. Apps can run either stand-alone or as Jack plug-ins. Claims to be suitable for professional work.
Pure Data, Miller Puckette's next step in the evolution
of the ideas in his MAX program.
A standard library of objects for PD.