You should check out OpenAL. It's got an open licence, but it will take a bit of work to set up (SDL takes care of a lot for you).
On a side note, I used SDL for a while and found SFML. It's a modern, OpenGL, open-sourced library that has pretty much everything SDL has. Portability is not quite as extensive (you won't be running SFML on a portable device or console), but it's definitely worth checking out. SFML has a MUCH more feature rich sound system. When I switched to SFML I never looked back to SDL.
Also, I'm new to these forums, so I'm sorry if "advertising" is against the rules, haha.