The patch at https://reviews.freebsd.org/D6183 fixes the "ioctl sign-extension ioctl ffffffffc0044d00" warning that appears in dmesg all the time when using it and spams the console.
A commit references this bug: Author: pi Date: Wed May 4 05:49:35 UTC 2016 New revision: 414583 URL: https://svnweb.freebsd.org/changeset/ports/414583 Log: audio/umix: fix the "ioctl sign-extension ioctl ffffffffc0044d00" warning PR: 209227 Submitted by: trasz Changes: head/audio/umix/Makefile head/audio/umix/files/patch-src__driver_oss.c head/audio/umix/files/patch-src_error.c head/audio/umix/files/patch-src_error.h
testbuilds are fine, committed, thanks!