mistergreen I'm surprise you need your engine on Unity since sound on Unity runs through a virtual sound engine that adapts to whatever platform.
When you have a full synth/modular synth and sequencer/automation audio engine, you know why. 🙂
Well not all audio C++ is written the same, you have to write it multi-platfrom from the start.
Edit: Unity has a low level audio callback as well, that is what my engine could hook into also but I don't ATM. I use OSc messages from abstraction and call out to a native plugin. All my bootstrap code stays the same, I will probably hook the processAudio() into Unity's callback sooner than later (then my audio stream is in Unity's AudioMixer graph).
Then... procedural track creation...... ah the fun in 3D then.