Valve have just launched Steam Audio a new SDK for advanced audio processing in games. Currently it ships with support for a C api and with a Unity SDK, although a Unreal Engine SDK is in the works. In Valve’s own words, Steam Audio is:
Steam Audio delivers a full-featured audio solution that integrates environment and listener simulation. HRTF significantly improves immersion in VR; physics-based sound propagation completes aural immersion by consistently recreating how sound interacts with the virtual environment.
Essentially it enables you to create realistic real-time audio simulations, by integrating audio processing into your 3D world. You define sound sources within your 3D world, define what geometry does and does not interact with the audio simulation and their acoustical properties and finally define the position of your ear in 3D space. Steam Audio is then capable of simulating more accurate audio positioning in a 3D world. In many ways it works a great deal like existing physics engines, except for audio. Obviously its tailor made for VR applications, but is useful for regular 3D applications as well. Currently Steam Audio supports the following platforms:
There is obviously one major missing platform... iOS. It will be interesting to see if it is added in the future. Somewhat surprisingly for Valve, this is not an open source project. It is however free to use and is available for download here as both a Unity package or C API.