UKSound and UKSystemSound
Because it came up on a mailing list, I've made available two Cocoa classes I wrote for an iPhone application. I think these classes should also work on Mac OS X, but I haven't really tried that yet.
The two classes are called UKSound and UKSystemSound, and are intended as replacements for NSSound. The delegate methods and initializers and methods are as similar as I could make them. Note that both these sound-playing classes are very simple, basic implementations. They just let you play a sound and find when it has stopped playing. You don't get volume controls or a call to stop playback at this point.