Visiting https://www.freebsd.org/cgi/man.cgi?query=sound&apropos=0&sektion=0&manpath=FreeBSD+10.1-RELEASE&arch=default&format=html should result in seeing the man page for sound(4) yet nothing appears. The man page is present on a clean 10.1 system w/ docs installed.
I've fixed this. Thank you for the report.