JUCE:
http://juce.com/
Tracktion:
http://tracktion.com/
Both very powerful audio frameworks - JUCE does plugins and audio drivers and low-level DSP, oh my - and Tracktion does all the stuff a DAW needs, on top of JUCE.
There are tons of ways to contribute, from building open source samples, to testing, or even adding functionality. Both dev teams are open to good quality PR's being submitted and both frameworks have excellent communities that will get you started: http://forum.juce.com/
These are cross-platform tools which offer Audio developers an extremely powerful toolset. By contributing to either (or both) frameworks you will be massively contributing to the audio world - so many plugins use JUCE these days!
EDIT: see, also, awesome-juce - pick a project from this list to contribute to, according to your interests ..
https://github.com/sudara/awesome-juce