We are looking for a good 3D sound engine for a game written in C#. We know of irrKlang, which looks great but isn't free for commercial uses, nor is it open source. XNA has an alright one, but it binds us to DirectX and therefore, windows. Are there any other good free ones out there?
FMOD and OpenAL are two very powerful alternatives to using DirectX directly. The Tao libraries include OpenAL .NET implementations (as well as OpenGL and other game libraries), but FMOD will need to be used via P/Invoke.
Additionally, OpenAL is free, and FMOD is only free as long as your application is free (see FMOD Non-Commercial License here).
If you want to sell your game but not pay for a 3d audio library, you're pretty much stuck with OpenAL.
It may cost money, but FMOD has an extremely well thought out API. Something else to consider is the added value of their EventSystem. It's not as robust as Wwise, but being able to define high-level sound events in an external authoring tool makes achieving high quality audio a lot easier. Stuff you would have to spend time coding is easily defined in terms of parameters and sound instances in FMOD.
If only they allowed you to add events as well as sound instances to event layers.
BASS from Un4Seen is another free-to-try, pay-to-sell library that supports Windows and OS X.