Basically, on OpenBSD, audio functionality was partially moved from kernel to user space, it was extended and called sndio. From this standpoint it's the OpenBSD native audio API. The remaining kernel audio(4) and midi(4) APIs are internal interfaces used by sndio to get access to the bare hardware; they are too limited to be useful to typical programs.

For the sake of easier application software, yes. sndio is the higher level of abstraction. There is a lower level API, audio/mixer/midi(4), but they exist as only ioctl(2) calls.