I'd like something that would work on windows/linux/osx, and be able to mix 3+ sound files at a time. Doesn't matter much what format, as long as it is one I can easily convert the source material to.

Do you need also loading or just mixing? Because mixing is just a loop with adding numbers together.
For loading and playback you can look at:
alure + openal - http://kcat.strangesoft.net/alure.html (open source)
irrKlang - http://www.ambiera.com/irrklang/ (free for non-commercial use)
Cricket audio - http://www.crickettechnology.com/ (free to use)
fmod - http://www.fmod.org/ (free for non-commercial use)
bass - http://www.un4seen.com/ (free for non-commercial use)
SDL_Mixer, if you already use SDL - https://www.libsdl.org/projects/SDL_mixer/ (open-source)

Thanks, I'll take a look at those. Yes, need loading and playback as well.