Well working within the concept that this is meant to be easy implementation of audio in a beginner's game(s), There should be an audio format or formats which has some basic compression and can be loaded and played by the game library. Rather than supporting all kinds of formats, why not just support one and have a tool that is capable of converting all the other formats into whatever container you wish them to use in the game library.

For example, DelphiX had it's own graphics and sound container formats, which it could load and then play back sounds or music from them as needed. It was simple and lots of people loved the game components for it. This approach could work just as well here so long as your tool can convert all of the most popular music formats into this one format and was able to pack them into your easy to manage container for use in games using the library.

This eliminates the hassle of multiple format loading and bouncing between audio headers and API. Just build it for one format, then create your format for an audio container. From there all you need to do is make the converter/container tool.

If you are looking for audio mixing, either SDL or OpenAL are both cross-platforms and the easiest to work with. OpenAL is 3D already and it's flexibility makes it very powerful to work with. It'll also take advantage of EAX hardware acceleration as well if it exists. Noeska makes the best OpenAL headers that I know of. http://www.noeska.com/doal/