Both has bindings for C, both can play various formats.
Which one is more superior? in terms of simplicity, performance, overhead and memory footprint.
Also which one is better at handling multiple streams?
I have not programmed with either of those, but I believe that OpenAL has been designed to render and output multiple-channel audio for games, with real-time performance as a requirement.
libSoX is more for input and output from audio files, as well as for format conversions. There are lots of plugins but AFAIK it has not been designed for real-time audio output. It seems significantly simpler to use, though.
You might also want to have a look at libsndfile.
What exactly is it that you want to do?