I tried making a
   phony /dev/audioctl device (i.e. made it the same as /dev/audio) to fool it,
   but alas, it tries some ioctl() that returns an error under NetBSD and thus
   it fails.

It's probably doing an I_FLUSH.  This is common in code written for
SunOS or Solaris.  It could be emulated by doing the same thing that
AUDIO_FLUSH does.