I've concentrated all the kernel patches we've made for the mixer here.
Having these patches available in 13.0-RELEASE can be useful for
people who want to try out the new mixer features on their
system without upgrading to 14.0 or compiling a 13-STABLE kernel.
References:
https://reviews.freebsd.org/R10:4a83ca1078e3ec70159741b51a6b48e844ada9f5
https://reviews.freebsd.org/R10:1483b121d0285bda728833e9690570ec6dbe3982
https://reviews.freebsd.org/R10:66e91a9ee19fe09125e5cf535c440c80faf77fa6