In case anyone else is experiencing a segfault in alsamixer in 6.3 with all of the updates, just download and install alsa-utils-1.0.22-5.el6.x86_64.rpm<http://ftp2.scientificlinux.org/linux/scientific/6.4/x86_64/os/Packages/alsa-utils-1.0.22-5.el6.x86_64.rpm> from 6.4, and things should start working again.

Jonathan