Yeah, I've searched for a while, but I am surprised I didn't find any cross-platform library to compute spatial audio. It shouldn't be so hard, right? The actual audio output could be provided by multiple libraries, but spatial audio calculations seems to be exclusive to OpenAL and FMOD (proprietary).
There are some obscure libraries able to compute HRTF audio (which uses a "model" of the ear to make realistic sounds for each channel, at least on headphones), but I also couldn't find a cross-platform library for that. The closest I got was the slab3d library (
http://slab3d.sonisphere.com/), that is licensed on the NASA Open Source Agreement, but it is said on the site that it is made for Windows, so I am not sure if it would work on other platforms. It also has a lot of features that are not neccessary for SFML.
There is a demo that someone made using it here (watch it with headphones):
https://www.youtube.com/watch?v=c4MBEoBHTf4By the way, if I understood the NASA license correctly, we could use the library (even with static linking) as long as the library's code and eventual modifications we do are released, and we can keep our own code closed. Would that license be acceptable to use on SFML?