Quod Libet [1] is a mature but "lesser known" audio project written in Python and GTK+ (3).
Whilst there are probably some quite high barriers to entry (the user community is very tech- and music-savvy, the project is fairly feature-complete, and the codebase quite complex), we're fairly welcoming and try to take care of our (large) backlog of tickets whilst welcoming useful contributions where possible, or at least discussing / explaining their considered rejection.
In particular, we could really do with a dev (or in fact buildmasters / testers) who are Mac-based ideally with some knowledge of building and testing Python and GTK+ apps on OS X. Probably best to email the list for further details.