Since 14.0-CURRENT, mixer(3) has been added to base system and mixer(8) has been updated. The web man pages for 14.0-CURRENT don't reflect that change yet. mixer(3): Web link: https://www.freebsd.org/cgi/man.cgi?query=mixer&apropos=0&sektion=3&manpath=FreeBSD+14.0-current&arch=default&format=html New man page: https://cgit.freebsd.org/src/tree/lib/libmixer/mixer.3 mixer(8): Web link: https://www.freebsd.org/cgi/man.cgi?query=mixer&apropos=0&sektion=8&manpath=FreeBSD+14.0-current&arch=default&format=html New man page: https://cgit.freebsd.org/src/tree/usr.sbin/mixer/mixer.8
@wosch, maybe the man pages for -CURRENT are outdated?
After an update of the -current manual pages the link works now.