OSS version 4 from http://developer.opensound.com/ is supposed to allow multiple sound applications to play simultaneously, and it has improvements over previous and forked versions. For the most part, it has a BSD license.
There is also sndio, http://www.sndio.org/, from OpenBSD, which can handle MIDI frontends to a sound server (or directly to the hardware). It would be nice to see the newest version of OSS put into NetBSD, with layers of sndio to bridge MIDI and API's to OSSv4. Also, across all BSD's, there is not a simple drop-in BSD replacement of libcanberra, to act as an API from certain applications to OSS or sndio. Thank you