src.opts: Introduce MK_SOUND

PR: 291853
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: zarychtam_plan-b.pwste.edu.pl, markj
Differential Revision: https://reviews.freebsd.org/D54456

(cherry picked from commit f74f891581bc4df20431607de9b761107f6a0aa7)
(cherry picked from commit 37164224eefd11278b0203e1367e1d9d1454da3f)
(cherry picked from commit 241a43cf9da8933f500e7d0df7fff621fd52168c)
(cherry picked from commit cf34eb8264954a8fd1e92081bcf67719bddbc2aa)
(cherry picked from commit 466bad427d74e666e85445729b2e63b82e10d18f)