An mmap'ed audio driver sounds pretty weird to me.  Yes, you can
   make use of it to reduce the kernel/user data transfer, but you
   can do that the other way around.  [...]

That's true only if the samples you're playing are already in a file.
(And even then, you could just do memory-to-memory copy if both things
are mapped.)  In other cases, like xquake B-) and AudioFile, this is
not the case, and eliminating the copy time is actually a big win.  One
of the AudioFile authors specifically asked me about this, in fact.