Ahead of this years Game Developer Conference, two of the main audio middleware companies announced they’re making their SDKs free* for indies:
http://www.develop-online.net/news/audio-middleware-fmod-studio-goes-free-for-indies/0190187
The catch is that for FMOD your budget has to be under $100k, and for Wwise your project has to contain less than 200 sound files.
It would be great to see support (plugin or otherwise) for these audio solutions in Corona!